Do you think Len ever thinks about that night in the bar? That time Mick called him up out of the blue and asked to meet for drinks and then acted really weird, told him he was a hero and left? He probably didn’t think much about it when it happened, Mick has always been a little off. Shrugged it off and when Mick didn’t mention it the next time they saw each other he forgot about it.
But fast forward three years to January of 2016 when a man with a cowboy fetish shows up and tells him he’s meant for great things if Len will just come with him on his time machine spaceship. Maybe that night comes back to him in a dream, he wakes up a little dazed with neon and girls dancing in his vision and Mick’s gravely voice telling him he’s the greatest man Mick ever knew. He looks at Mick a little funny the next morning while Mick shoves disgusting sugar free future muffins in his mouth, but doesn’t say anything.
However as his time on the Waverider goes on Leonard Snart begins to realize that maybe that night meant more than he’d originally thought. And by the time he and his team (friends) are at the Vanishing Point and Ray of Sunshine Palmer is trying to give up his life for his friends Len knows. He knows he’s not going to make it out of this one - he recognizes the Mick from that night as the Mick from the Waverider and the only reason he would have been in 2013 telling Len he’s a hero is if he didn’t have any other way of telling him.
So in the end it’s easy. To knock Mick out. To pry the ring off his finger and plant it in Mick’s jacket. To make sure his best friend and the rest of the group that he may consider friends are out of harms way while he holds down a failsafe. When the light from the Oculus explodes around him it’s easy.
Because he knows Mick survives to see him again in a cheap bar in 2013.