AURORA, Colo.— For nearly three years, Tom and Caren Teves have ached over the murder of their son, Alex, in a mass shooting at a Colorado movie theater—grief made worse by the fact that there has been no reckoning for the accused gunman, James Holmes.

But after numerous legal delays, Mr. Holmes is set to stand trial Monday. He is charged with killing 12 people and injuring 70 during a July 2012 screening of a Batman movie, “The Dark Knight Rises,” a case that will largely hinge on the former graduate student’s mental state at the time of the shooting.

“It is ridiculous that this thing has had to hover over our heads for as long as it has,” said Mr. Teves, who along with his wife has flown in from out of state for the trial. “You never get past the fact that everything hangs in the balance of whether Alex will get justice or not.”

The Teves family is steeling itself for a lengthy trial that will rekindle anguished memories. They are part of a large group of survivors and victims’ relatives planning to attend opening arguments at the Arapahoe County Justice Center in the case of one of the bloodiest mass shootings in U.S. history. Some of the 70 injured also are expected to testify.

In addition to being emotionally harrowing, the trial, which is expected to last until Labor Day, is also legally complicated. Arapahoe County prosecutors are seeking the death penalty. Mr. Holmes, 27, who faces 166 counts of murder, attempted murder and other charges, has pleaded not guilty by reason of insanity.