Thread:PrimalFan/@comment-29421682-20171120000826/@comment-28063203-20171126043508

Oh, the red-and-black suit is the main suit up until he gets the symbiote suit.

If you want a more detailed (but also has spoilers) reason:

Basically, during the first three seasons, a theme that reoccurs for Spider-Man is moral ambiguity. Multiple times in the series he nearly crosses the line of killing the Green Goblin, and one episode is devoted to him snapping and almost killing the Green Goblin (but indirectly kills the Green Goblin anyway...).

The red-and-black suit represents his character, struggling with the powers he has and unsure when or if he even should use it to kill. The symbiote then basically amplifies these temptations, and Spider-Man after seeing Dennis Carradine has exited jail the symbiote starts to cause him to try and kill him and Spider-Man doesn't fight back and is willingly going to kill him but then he stops when Jean DeWolff and Stan Carter appear and then his reputation starts to plummet. He removes the symbiote, and the symbiote carrying the emotions from Peter amplifies Venom's and Peter feels responsible for the creation of Venom. In everyone's eyes, the red-and-black suit isn't a suit that symbolizes the hope Spider-Man brings but a suit symbolizing terror.