Is it Illegal to Hack Into Someone's Minecraft Account?

Just saying first: I wouldn't access any actual money accounts. Just in-game stuff. But I was wondering, I play on this Minecraft server and this one dude who no one likes (It's a small server) always kills everyone. I was just wondering if it would be legal to hack into his account and get the stuff back. He always wins it unfairly and he has hack clients but we haven't been able to prove it fully. As I already said is it illegal to do this? I WOULD NOT access any money accounts or anything of the sort.