when you only do yourself everything without someone's opinion or approval if even you join a group or when you have never confirmed with anyone else.
it is similar to a selfish behavior.
he used my computer by his decision without my permission.
it can express like that かれは、”勝手に” わたしのPCをつかった。
if i say this situation.
he has never referred anyone's opinion,if even he consulted them.
moreover he is really stubborn.we say it ”勝手にしろ！”勝手にすれば（いい）。 *勝手にしろ／suit yourself ,do as you like...