One night, Mick Rory fell in love with his father's lighter fluid and started a fire that he couldn't control. It got out of hand and Mick panicked in fear, running away without waking his parents or trying to save them, he left them to burn and watched the flames instead, completely hypnotised by it. Mick has always hated himself for what he did and in many ways, it caused a lot of self loathing and guilt that have plagued him his entire life.
Mick was always a delinquent and that's how he eventually wound up in juvie around the time he was sixteen, and that was where he met fourteen year old Leonard Snart. He saved him from getting himself killed by some older inmates and since that point on, the two forged a strong friendship. Mick as the protector, Leonard as the brains of the operation. They worked together, on and off, since then onwards until an incident in 2014. They were working a job that went horribly wrong and over half of Mick's body got burnt when he lost control of a fire. It's implied by Leonard that Mick lost focus and got obsessed with the fire he started, a habit of Mick's. After being horribly burned, he escaped from the ambulance on the way to the hospital and disappeared, probably to lay low.
The botched job split Leonard and Mick up for a year, it was only when Leonard approached him again, this time as Captain Cold, and offered Mick a chance to join up with him. He gave him the heat gun and decided to bring him in on his rivalry with the Flash. For months, they terrorised the city as Heatwave and Captain Cold.
It was a whole year later that Rip Hunter entered their lives. He made them an offer to come with him and join Rip on the Waverider for a time travel adventure fun time extravaganza with a bunch of other
They continued on, helping the team and doing missions together but things took a bad turn when they went to Star City is 2046. It was there that Mick found somewhere he truly wanted to be. Instead of getting to stay, Leonard knocked him out and forced him back to the Waverider. Mick knows Leonard is 'the boss' and Leonard knows it too but neither ever stated it for that exact reason - because suddenly their power dynamic was called into question. Mick felt betrayed by the way Leonard had acted.
Mick was suddenly at a loss, everything he knew had changed, his role was different and he didn't even have Leonard to fall back on any more. All he wanted to do was go back home to 2016, a time he understood, taking Leonard with him and reclaiming what they had before. It didn't happen. When Rip told Mick he was only brought into the team because he was a package deal with Leonard, this caused Mick to snap and he betrayed his team by helping a group of space pirates over-take the waverider. The plan was to steal the ship and return to where he belonged, offering for Leonard to join with him but when he was denied, he decided to fight. After his defeat, the team could no longer trust Mick and he was deemed too unstable and dangerous to be left on the Waverider. Leonard decided to handle it, taking Mick to the middle of nowhere and dumping him there, threatening to kill him.
Leonard had informed the team that he'd killed Mick but in reality, he dumped him in the middle of nowhere. Sort of like an ill-thought out time out. He had every intention of coming back for Mick but as far as Mick knows, he's stranded.