For complicated reasons that aren’t worth going into, Beast is in Canada (where Hulk ran to after last issue) and he’s trying to help Mimic. Hulk runs across Mimic in a cave. Hulk and Beast end up fighting because he’s Hulk and he fights everyone.
Mimic wants to save Beast so he absorbs Hulk’s gamma radiation, which kills him.
Or at least it seems to. He’ll be back.
Anyway, this means Hulk killed him. At least under the felony murder rule: He was needlessly fighting Beast, probably with intent to kill, and Mimic interceded and was killed in the process.