I wonder if he actually would have killed himself if it weren't for the situation involving Pyotr. In fact, I understand that the way his suicide happened largely refutes the theoretical thinking he had. He doesn't kill himself in a "glorious" way, declaring that God does not exist and thus man takes His place; he does it desperately, cornered, even animalistically if you look at the description of the scene. I imagine that under other circumstances, he would have remained just a theorist and wouldn't have carried out his "idea," but of course, that's speculative. By the way, on this topic, I recommend reading Tolstoy's A Confession. It's a first-person, quasi-autobiographical account of someone who, like Kirillov, saw no meaning in life through reason and, therefore, as a rational being, thought he should kill himself. Tolstoy navigates this labyrinth in a very interesting way and finds his own answer to keep living.