I'm not sure whether I can give you a thesis sentence, but I think that what you ought to say in such a sentence is that both of these young men were dangerous characters but they wouldn't have committed the terrible crime they did if they hadn't been brought together like two chemicals in a testtube. It is often the case that two people will do things that neither of them would do separately. I think you can see in the book how they both seem to be trying to impress each other with how bad they are.
It occurs to me that Shakespeare was demonstrating the same truth about human nature in Macbeth. Lady Macbeth could not have committed the murder of King Duncan by herself, and Macbeth pretty obviously would not have committed it if his wife hadn't kept encouraging him to do so.
You should use Perry's and Dick's full names in your thesis sentence.
Like certain chemicals, antisocial people who are relatively harmless by themselves can form a lethal partnership when they are thrown together; this is what happened with Dick Hickock and Perry Smith.
That is my suggestion for a thesis sentence for you to tinker around with. I believe that Truman Capote felt that way about the two killers and may have actually said something like that in his book.
In the famous Loeb and Leopold murder in the 1920s, the two young men would never have committed the crime separately, but somehow they became lethal when brought together. And the fact that there were two of them made it possible for the police to use one against the other, just as they did with Dick and Perry.
good idea for thesis sentence