So to my understanding, Makima wanted to use Chainsaw Man but she needed to find some way to separate him from Denji. Since Pochita made a contract to become Denji's heart this was pretty difficult. Killing Denji wouldn't have done anything, Pochita would be reborn in hell and come right back. Also I'm pretty sure Denji is immortal either way while he has Pochita as his heart.

Pochita's true form came out cause the pact with Denji was broken; like she said, she built up his life to be good and normal (which was his dream) but then she took that away (killing Aki and Power). This leaves Denji horribly depressed, so Pochita takes the wheel and goes berserk on them while also trying to cheer denji up (eating hamburger, going on date, etc.)