This behavior was spurred from his early years in which Carnage faced persecution for both being black and a wizard, and thus neither getting solace with either white humans or black humans during the fight for equality. This all weighed heavily on Carnage until he was eventually driven insane and killed 27 people and crippled 5 at a White Castle in Newark.
Carnage's school of magic is based on the elements and invoking things - hence the name. High-ranking wizards in this school can shoot fireballs, lightning, make acid, all the usual gimmicks.
Carnage says hello to his friends by setting them on fire. It is customary to scream EXACTLY for 5 seconds and then roll in a oval along the ground to say 'hello' back.