He is Lammy's possibly imaginary friend who has a mischevious and dangerous personality. As such, Lammy is the only character who actually sees Mr. Pickels as a living creature. Anyone else would just see him an ordinary pickle. Mr. Pickels causes mayhem by killing/murdering characters. When Lammy tries to stop him, it would appear to someone else that Lammy caused it, thus making her the scapegoat for Mr. Pickels' crimes. It is possible that Mr. Pickels wants Lammy's attention all to himself, so he kills anyone who interacts with her, causing other characters to stay away from her, thinking her as a psychopath.
As Mr. Pickels seems to be a figment of Lammy's imagination, it may be that Lammy is the one who actually commits the murders. However, if he is imaginary, it is unknown how Mr. Pickels is able to change his locations. If he is imaginary, it is also unknown how he managed to kill Flaky single handedly in Royal Flush.