You know what's especially funny? When you are playing as a Soldier and go to rocket-jump, and end up killing some poor, unlucky soul who just happened to be nearby. Generally, if I am put into that position, I already have low health from the heat of battle. The resulting rocket, more often than not, kills me.
If I manage to take someone out with me, more power to me. If not, I've made a mockery of myself and I bet it was rather funny to watch from my teammates perspective. Ah well, off to respawn and then back into the fray. Blowing myself up usually comes by accident, but when it does happen on purpose, I just scoff at the screen and smile to myself.
Usually exclaiming "Well, that was dumb." A kill is a kill in TF2. That's a few precious seconds you have bought your team from harassment of that player you just killed. I'd consider it a "net gain," as you so put it.