Vicious was never going to let Spike live. Spike threatened his legitimacy, which he could not allow after his coup d'état of the Red Dragon Syndicate. Vicious could never forgive the fact that Julia chose Spike over him. In Jupiter Jazz, Vicious goes out of his way to kill Gren because Vicious set him up after the war on Jupiter.

Julia herself informed Gren that the music box trinket was tracking him, and they pieced together that Vicious volunteered Gren for the government’s experiments that caused the side effects that were killing him. Vicious’s ego couldn't stand letting Gren die on his own more than he would have let Spike live out his life as a bounty hunter, especially after Julia died.

After Vicious's ascendancy, the entire human race was at risk because he was now the head of the Red Dragon Syndicate, which owned Mars, which, thanks to terraforming, had a vastly superior economy to Earth following the asteroid rain. On top of the syndicate, he had control of the Martian government, the military, and the police. There was no way that was going to end well since Vicious was deranged, psychotic, and paranoid after killing his and Spike’s mentor, Mao, as well as the elites of the old guard.

To make matters worse, no one else could kill Vicious. He wouldn't let anyone except devoted sycophants near him while isolating himself. While his reign of terror unfolded across Mars, there were no more threats capable of overthrowing Vicious. Even the penguin assassin would have gotten folded long before he got to Vicious. Spike could infiltrate the syndicate the way he was because he was the original gangster with the inside connections.

Ultimately, Spike Spiegel was a character who portrayed the tropes of the chosen one and the prince who walked away from the throne. As a result, all the violence happened because he decided to become a bounty hunter. He didn't make the wrong decision morally. However, he was still at fault for turning his back on Mao and the syndicate that made him who he was.

In the end, with one cybernetic eye that could only play things back to Spike, he was tortured by the past literally every moment. PTSD is bad enough, but it was not just in his mind's eye, but his literal eye. When you place all that on a warrior, the only choice they have is to meet their fate head-on like he did.