Lobo is a space bounty hunter and merc, usually the people he's hired to kill are those that deserve it.
To really piss him off is to bring death and destruction to vast swaths of whatever solar system hes in at the time.
Only the most resolute various color corps members constructs can hold up against him since his willpower, amount of fear he causes, rage, love, etc just overpowers most average joes.
Only way to stop Lobo is to contain him and even thats only a temporary measure since he'll dedicate his entire being into busting loose and getting ahold of who stopped his party.
Not to mention he's still a high ranking L.E.G.I.O.N. member and Vril Dox is more than willing to make a mockery of the GLC to the universe at large (see the issue of rebels where Lobo beats up 2 rookie gls and then vril makes the glc into villains for the universal news channels.)