Assuming someone does istikhara for a certain situation. And the istikhara comes negative to a certain decision. Yet still the person goes about going for his intended decision. Would this be considered a sin as going agaisnt the decision of Allah?
Re: Istikhara results...
Istikhara is not the "decision" of Allah. Rather it's to be taken as optional guidance. Whether you choose to follow it or not, is entirely upto you. Istikhara in spirit is meant to be done when you're not sure of how to go about something, and add to that your logic.
Do what you think will be best for you, start with Bismillah, and inshaAllah everything should turn out good if you've done your homework.