What do I do?

Okay, two days ago my son set the recliner in the living room on fire while I was taking a shower...I knew something was up, cause they were being quiet so I got out and my chair and part of the carpet was on fire. Don't know how he did it, it was a safety lighter for the fireplace, and he must have climbed up the table or something to reach it cause it was on the mantle behind a picture. Anyway, after I put it out and made sure he and Bella were not hurt, I spanked him, and put him in his bedroom, he knew he was in trouble bad, he cried himself to sleep, Ejay came home, and we both talked to him. He knew what he did, and that it was bad, and he, or Bella or all of us could have gotten hurt. So What do I do? What did I do wrong that he is so roudy, and misbehaves, and I don't know what to do anymore. I am so lost. I need help. I thougth maybe the spankings aren't working, so today when he ran away from me cause he didn't want to come inside after I let him play outside for an hour, I locked him in his room for a time out, Yes, locked, cause if I just sit him in the corner or in a chair, then go to take care of something for Bella or something, he justs gets up and starts all over again, anyway, after I put him in there he starts throwing an even bigger fit and banging and throwing things at the door, I told him he had to be quiet to come out of his room, and after about 15 minutes or so he knocked on the door and asked to come out, then we had a talk, again, but more calmly or his part about why he had to go to time out, and he has been pretty good since. So maybe I just have to do that, I don't know, truly I don't. I have to do something, cause he can't act like this, especially at school, and he starts kindergarten in August....gosh, think about all the principals phone calls I'm going to get....I'm tired, I'm stressed, I feel like the worst parent ever, and I don't know what to do anymore, I'm just lost.
Help....and I don't need any negativity, I already feel bad enough.