An excuse for what? He didn't directly kill half the universe. If Star Lord were a real person it would be wrong to criticise him for having emotions. The moment you do that, the moment you have the Cybermen from Doctor Who taking over mankind.
Nobody here have criticized Star Lord for having emotions, we have criticized him for dooming the universe.
No matter how you are feeling or what emotions you are experimenting, you have complete control over your actions, and can (and will) be juzged accordingly.
If you kill someone (or are responsible for the premature death of someone) and your only legal defense is "im a human, i have emotions" bad news, electric chair awaits you.
Or, if you are actually saying the truth and you actually lose control of your own body when you are experimenting emotions ( you want to move your arm in one way but you can only watch in terror as your body moves uncontrollably without responding to your will) then you should go to your doctor and/or psychiatrist ASAP because something is terribly wrong in your mind.
You cant control your emotions, but you CAN control your actions.
We arent judging Starlord emotions, we are judging his actions.
You can be extremely mad with someone, and wanting to kill him/her or just beaten them to a bloody pulp, but you arent forced to anything, you can choose whatever you want to do, and, then, you can and will be judged for it.
Star lord is responsible for the death of billions if not trillions of lives, and that makes him the biggest criminal in the history of mankind.
And no, as any lawyer would told you, having emotions doesnt justify anything, you are responsible for your actions.
To sum up:
Morally, logically AND LEGALLY, you are responsible for your actions, no matter what emotions you were feeling in that moment.
No matter what are you feeling, you can control your actions, even in the worst case scenarios.
We arent judging Starlord emotions, we are judging his actions