That its *LINUX* that did this?
No, its lazy / lax security. And assuming you'll be fine, just install it you are all good.
Nope, its just plain old stupidity there. PLUS, it still only had the permissions of the user running the game's IRC server. Trojan'd IRC servers are nearly the oldest Trojans out there.
If you run those services as ROOT, you are just asking for trouble. I run all services as a user, except for those that spawn correctly to a runas user because they have to be able to open a port below 1024.
Come on... this is SIMPLE. Trojans are as old as computers and have been available on every platform since the beginning of time. You have to get them from a trusted source.
This is known bullshit and Lax security and lazy admins.