It wasn't until well after they were caught that Malvo made his jailhouse doodlings. I think that had more to do with the fact he wanted to believe he was a part of something larger and more important, rather than just being a criminal. The fact they were converted muslims likely fed into their already disgruntled and murderous personalities, but no evidence points to it being the main reason for their crimes.
The snipings and panic is caused was certainly terrorizing. However, they were no more terrorists than University of Texas sniper Charles Whitman was.
But one must acknowledge that many terrorists finance their terrorism through petty crimes.