He is not an ordinary criminal. He is not a criminal at all. A criminal will fell guilty inside the mind.
For a Jihadi, killing non-believer is as equal as trowing a flower for worshiping GOD. It is a work of GOD for him.
Many say that evern muslims are victims. The Jihadi believes that if an innocent muslim is killed, they will goto jannat as he got killled in a GODly act of killing non-believers.
It is a big mistake to think that terrorists are like any other criminal or paid killers. That is why even fully educated muslims can become JIhadis.