In truth, this depends on what canon you reference. In the Rob Zombie remakes, Halloween (2007) and Halloween II (2009), Michael's home life is extremely abusive and toxic. He endures an absent father, a mother who's in a relationship with an abusive man, and an older sister who bullies him. He commits his first murder at age 10 (rather than six like the original), and he doesn't stop at just his sister. He also kills his sister's boyfriend and his mother's abusive partner before he's apprehended and taken to the sanitarium. Through this sequence of events, Zombie's remake attempts to humanize Michael and give him a sympathetic motivation that the original film never offered. It frames Michael's motivation for his initial murders as an attempt to put an end to the years of torment he'd endured, creating a killer in the process.
That's the problem with the Halloween remakes, though. The truth is, the original Michael had no motivation to kill. We do not at any point see an underlying reason for why he kills. Michael doesn't exhibit emotion, and the more we watch him, the less human he seems. That's the point of "The Shape." He is extraordinary in every way that an ordinary person would find horrific. Some argue that killing is the only thing that makes Michael feel anything. In Halloween Kills, Laurie even says, "The more he kills, the more he transcends."