Labour’s former deputy leader Tom Watson has said he left parliament because of the “brutality and hostility” he experienced within Labour and blamed poor organisation and messaging for the party’s disastrous general election result.

Watson, who stood down as an MP and deputy leader last month, condemned Jeremy Corbyn’s advisers and told the Guardian that Labour’s next generation of leaders must address whether they “actually want power” as they begin to rebuild the party. In an admission that will draw criticism from Corbyn’s supporters, he also said he had voted for the leadership challenger Owen Smith despite his position as deputy leader.

Watson’s stinging criticisms, in an interview with Weekend magazine, are his first intervention in the debate over the party’s disastrous showing at the election earlier this month, in which Boris Johnson won a majority of 80. Watson’s former West Bromwich East seat was lost to the Conservatives for the first time.

Corbyn’s critics have identified his unpopularity as a key factor in the election result, but figures on the left of the party say the defeat was primarily because of the difficulties over Labour’s Brexit position and not strategic errors or the leader.

In a succession race interrupted by Christmas, only two Labour MPs have formally declared their intention to run for leader: Emily Thornberry, the shadow foreign secretary, and Clive Lewis, a shadow Treasury minister. Other expected candidates include the two favourites, the shadow Brexit secretary, Keir Starmer, and the shadow business secretary, Rebecca Long-Bailey. Lisa Nandy, David Lammy and Yvette Cooper could also stand.

Watson said: “Does the Labour party in its current form actually want power? The ultimate betrayal of working-class people is not to take power when you can, and if you are a party that believes in power through elections, then that requires pragmatism, prioritisation, compromise and collaboration.”

‘So I just thought: now’s the time to take a leap, do something different. You’ve had a good innings.’ Photograph: Ben Quinton/The Guardian. Location: courtesy of the trustees of St George’s Hall, Bewdley.

Even though he was renowned in the party as a savvy organiser and occasionally brutal scrapper, Watson said he left in part because of the aggression he had faced within Labour. He said at one point police told him that a Labour supporter had been arrested for making a death threat via the party that Labour officials did not inform him about.

In a wide-ranging interview, Watson praised Corbyn personally but said conditions within Labour had contributed to making his political career unsustainable. “The point is that the brutality and hostility is real and it’s day to day,” he said. “So I just thought: now’s the time to take a leap, do something different. You’ve had a good innings. You’ve done good stuff. Go now.”

Watson cited the pressures of social media, factionalism and criticism from unions: “On their own, you deal with them and they’re a normal part of life. Combine them, and you’re carrying a very heavy load. And sometimes you’ve got to realise when that balance of life shifts and there are other things that are more rewarding.”

At what turned out to be Watson’s final Labour conference as deputy leader in September he faced a motion from the left of the party seeking to abolish his job. It was eventually withdrawn. Watson said that even amid the wranglings, the move surprised him: “I don’t think you could pre-empt such political idiocy and collective self-harm.”

Speaking immediately after the election defeat, Watson cited Brexit, Corbyn’s leadership, issues over antisemitism and a lack of party unity as reasons for the poor showing, as well as a convoluted election offering.

“I don’t even know what the message of our campaign was,” he said. “There were announcements everywhere, but none of them got through because there were so many. You knew what Boris Johnson’s was: Get Brexit done. What was the Labour strapline?”

Corbyn and his shadow chancellor, John McDonnell, have said they will step down following the crushing defeat on 12 December.

Watson also revealed that when Owen Smith challenged for the Labour leadership in 2016 following a vote of no confidence in Corbyn by Labour MPs, he voted for Smith.

“I did vote for Owen, but I’ve never said it publicly before,” Watson said. “I thought, as soon as the leader loses the confidence of the parliamentary party it’s almost impossible to see how you can form a government. I thought Jeremy should have resigned, and he nearly did.”

Watson’s departure from politics was also part of a wider shake-up of his life in which he has lost eight stone in weight and begun to retrain as a gym instructor.