[@PKMNB0Y]
In terms of motivation, I'd say its a mix of getting to fight dudes from across reality and necessity. Akuma might be an angry fightmonkey, but he isn't stupid, and can recognize when there's a greater evil to deal with (as seen with M. Bison). Since reality going kaput would ruin everything for everyone, himself included, he'd have his reasons to stick around and be quasi-heroic.

As for picking fights with other people, it's shown that he often seeks to push others to improve and get stronk, as seen with Ryu. So he'd likely limit it to the occasional sparring match to help them git gud, and reserve any duels to the death until after the kerfuffle is settled. He'd also not bother with cripples (*cough* Artorias) because muh honour.

Given that Akuma only did the whole island-busting trick after losing a fight, I'd figure the full-power version is more of a one-off desperation move than something he can throw out whenever he wants. The regular version (i.e. the one used in the games) is probably more around small house level.

Yeah that works.