Please help? how to get my best friend out of my life?

we have been best friends for 15 years now. we were in school together and grew up together. he was my club buddy and drinking pal and my drunk hookup. we did almost everything together. well i found out he has a side i never knew about. he hid his gang affiliation from me. how did i find out? he was arrested for 3 counts of murder. im still shocked. from what i heard he killed 1 guy because he owed him money and he killed the other 2 because they were with the 1 guy. he shot them all in the head. i never knew this because he was a quiet, shy white kid who didnt even look like he was a gang member. he dressed nice and just didnt look like he would hurt anything! so my question, since it will be hard for me to see him in prison for the rest of his life, how do i just get him out of my life and stop thinking about him?