When I was in Elementary school, our teacher taught us more in relationship to "greater" and "greatest".
You use "greater" to compare two people:
Janus is greater than Mike.
but if you're comparing to a lot of people you use "greatest":
Janus is the greatest [person I know].
In this case "more likely" relates to greater while "most likely" relates to greatest.
Because "more" is used when comparing two people, you would say:
Janus is more likely to commit crime than Mike because Janus has a history of mania.
However, if you wanted to use "most likely" you would say:
Janus is most likely [in the group] to commit crime because Janus has a history of mania.
An easy way to remember is more ends with the "er" sound like "greater" and most ends with "st" just like "greatest". (It's caused a few spelling mistakes for me in the past, but it helped me to remember which one I wanted to use on tests).