He sealed himself inside of the machine, knowing he wouldn't get out. I think he even refers to it as his coffin in his ravings. The only way he was getting out of there, is if a crane lifted the fucking roof off. And that wouldn't happen unless the vehicle was disabled and the cops were comfortable approaching, with no fear of being shot. I absolutely buy the suicide story. Why did he have an arsenal in there with him? He did shoot some stuff. Random shit, power transformers and other stuff that nobody cares about. The only person he shot was himself. And he had his moment, he could've lit up the son of the concrete plant owner- that's who was in the construction vehicle that tried to ram Marvin at the beginning. Marvin knew who he was. Should've ended the little shit, preventing him from going on to behave like his father.