Board Thread:General Discussion/@comment-1255618-20131207105231/@comment-24204226-20131218051721

Well, considering that Raph had used Spike as a sounding board anytime he was angry with his brothers, particularly Leo (which explains Slash's later comment after smashing Leo away from him, "I've been waiting a long time for that!"), I'd say he is definitely directly responsible for creating Slash. That, mixed with the effects of the mutagen only made his vendetta stronger and more hostile than Raph could have imagined.