What these killers really want is power, even temporary, over others. They can't get it any other way except through the use of weapons.
You see the same sort of thing from leftists. The left are always going on about how they're victims of the powerful. The state's only real tool is violence, which the left are eager to use on others.
“That makes it difficult for them to enjoy the infamy. “
They write these long manifestos, prepare the YouTube vids and facebook posts, planning that the media will obsess over them after the killer kills himself. They would not do that for a simple suicide, but the pattern is fixed- commit a horrible crime with semiautomatic weapons then kill yourself, and we’ll show your manifesto to millions and talk about you for at least a week.
They know it will be done posthumously, and plan the worst event they can to meet the criteria of the mass media.
I agree that power is part of it, but most of these killers have a deadpan look while they are working their evil, not faces of rage or even enjoyment. Any power they wield in this world is over in 20 minutes, but CNN coverage will be hundreds of hours.