I think its more important that the death serves a purpose. If you create a character just to kill them off, then I think you loose your readers a bit. I think the more important question might be why is the character being killed off, rather than the how unless your comic is a murder mystery... the how might be more important. XD
In my comic several people die, but each death drives the story forward in a very particular way. One death is accidental, one is due to negligence and the other is a murder. Each death shows a slow deterioration of a particular character that is pivotal to the plot.
Although here are some ideas if you are trying to kill someone off in your comic and can't figure out how:
*Betrayal- poison/stabbing/getting pushed through the moon door
*Accidental- attacked by animal, fell on knife, hit by car, caught in explosion, drowning, etc.
*Murder- go watch CSI, the possibilities are endless...
*Circumstantial- illness, old age, sacrificed self to save others, turned into a tree, killed by white walkers
good luck!