Sometimes the right thing can be done by the wrong person. Donald Trump’s bombing of a Syrian airfield seems to belong in that category, though even that verdict depends on events yet to unfold. For one thing, we don’t yet know if the 59 Tomahawk cruise missiles that rained down on the Shayrat base in the early hours of Friday morning were a one-off or the start of something more.

Antony Blinken, who served as Barack Obama’s deputy secretary of state, recalled that the US intervention in Libya, which he backed, began with a very narrow, legitimate goal – the protection of civilians from an imminent threat of slaughter – but “ended in regime change”. Blinken warned Trump of the dangers of “mission creep”, urging him “to avoid falling into an escalation trap.”

But let’s say the Shayrat strikes are not repeated. Given that the century-old prohibition on the use of chemical and biological weapons is a rare and valuable taboo, one that crumbles if not enforced, it’s hard not to welcome an act of enforcement. As Robin Niblett, the director of Chatham House, told me: “There are so few norms that are considered sacrosanct. If you don’t enforce this one, you create a sense of global anarchy, a global free-for-all.”

Reporting from the Syrian town of Khan Sheikhun by the Guardian’s Kareem Shaheen allows for little doubt as to both the human calamity of the chemical attack that befell that place on 4 April and where culpability lies. Shaheen’s eye-witness account leaves the Russian claim – that sarin was released into the air accidentally when Russian jets bombed a rebel-run chemical weapons plant – in shreds. There are some who still doubt that Bashar al-Assad’s forces were behind the sarin attack: they include US-based conspiracy theorists Alex Jones and Mike Cernovich, backed in the UK by Katie Hopkins, who uses the hashtag #Syriahoax. But their numbers are dwindling. The evidence points to Assad.

That still leaves a legal question. Trump acted alone; he did not have UN authorisation or even try to get it. Which means he might have been breaking international law in order to enforce international law. But that’s not the prime source of my discomfort. What troubles me more is that this necessary act was performed by someone who, in the words of radio host James O’Brien, you wouldn’t trust with scissors.

On Syria, Donald Trump has performed a U-turn so screeching, so dizzying, you can smell the burned rubber from here. Just 72 hours before these airstrikes, his administration was all but flashing a green light at Assad, hinting that he could do what he liked. Pull back further, and the volte-face is even more stunning. For years, Trump was adamant that he would stay out of Syria. Even when chemical weapons were used in August 2013, killing an estimated 1,300 people in Ghouta, Trump was firm: “What will we get for bombing Syria besides more debt and a possible long-term conflict?” he tweeted. It’s the abandonment of that stance that has so disappointed Trumpists such as Hopkins, Nigel Farage and the neo-Nazi leader Richard Spencer. They thought they were getting a true isolationist in the Oval Office.

Their mistake was to think Trump had a consistent foreign policy, rather than just a series of wildly contradictory impulses that can vary from day to day. Trump might well see this unpredictability as an asset. Recall how Richard Nixon encouraged Henry Kissinger to travel to foreign capitals, whispering to foreign leaders that the US president was unhinged. Nixon believed that if he were seen as a madman, capable of anything, it could only increase his leverage. He would be feared.

It’s not reassuring to think that the American president only acts when a tragedy hits primetime

In this context, North Korea and Iran may both be adjusting their calculus of risk. Now they know that Trump is willing to strike, with little warning. That he authorised the operation while at his Florida resort, where he was hosting the Chinese president, may have been an accident of timing, but it will please Trump. Think of it as a dominance display in front of a rival.

Above all, Trump will relish the comparisons with his predecessor. In 2013, Obama hesitated and havered over Syria’s use of chemical weapons, a Hamlet on the Potomac, his hand eventually stayed, in part, by Ed Miliband’s decision to vote down UK support for military action against Assad. Again, Trump was among those urging Obama to do nothing, further insisting that Obama needed congressional approval.

That scruple, along with everything else, will be forgotten now, as Trump revels in a comparison that, in his view, makes him look more decisive, more macho and even more humane than Obama. As Niblett says: “Trump has upheld a norm which Barack Obama, the great values president, did not.”

But that cannot alter the fact that, even as you welcome the act, its author remains wholly untrustworthy. Trump wanted us to believe he had been moved to action by the pictures of dead children in Khan Sheikhun. But what of all the “beautiful babies” killed away from the TV cameras these last six years, by bombs of a different variety? When they were being slaughtered, Trump was happy to shrug off their deaths, sending his secretary of state and his UN ambassador out just days ago to give Assad the wink that he could carry on as before. It’s not reassuring to think that the American president does not listen to his intelligence briefings or even read the papers, but only acts when a tragedy hits primetime.

But what makes his newfound compassion ring all the more hollow is that while Trump is ready to bomb a runway for those beautiful babies who are dead, he still won’t let America open its doors to those who cling to life. Refugees from Syria remain on Trump’s banned list, including every “child of God” traumatised by Assad and his barrel bombs, raining fire from the sky.

And forgive me if I don’t accept that this volte-face is quite as complete as the White House would have us believe. How convenient that Trump, under fire for being Vladimir Putin’s poodle, now stands up to him in Syria. How neatly this blows away all those allegations of secret links and election hacking. Yes, there have been ample statements of condemnation from Moscow, but those don’t cost either side anything. The US appears to have given Russia sufficient warning to ensure their men weren’t hit, and Russia used none of its ample capacity to hit back. It all worked out very nicely.

Some will believe none of this matters: the faded red line prohibiting chemical weapons has been painted scarlet once more. But intention matters, in foreign policy as much as in morality. There were those in 2003 who wanted to see the US-led invasion of Iraq as a humanitarian operation to topple an evil dictator. But humanitarianism was not what drove the architects of that mission: if human life had been the motive, they’d have heeded the warnings that they were making a terrible mistake, or at least planned for the aftermath. But they didn’t care enough to do either – and the result was catastrophe.

I didn’t trust Bush and Cheney, and I don’t trust Trump. I’m glad Assad’s ability to poison his own people has been reduced, if only a little. This deed is welcome. But I cannot applaud the man who did it.