This suggests that Holmes’s conversion is a defensive action against any regret that he may be feeling. But instead of meeting that regret with repentance, he has found a way to justify his actions. Instead of acknowledging his wrongdoing, he chose a belief system that justifies violence in certain circumstances, and has attempted to cleanse himself of any wrongdoing by seeing his murders in light of that belief system.
Jihad may take the form of indiscriminate mass murder of infidels, as we saw on 9/11, and 7/7, and in Beslan and Mumbai and Fort Hood and in so very, very many other places. Unfortunately for Holmes, however, jihad is not retroactive. If Holmes was an infidel at the time of the murders, then it doesn’t make his actions jihad to convert to Islam afterward.
Nonetheless, his conversion illustrates yet again why Islamic supremacist groups make such a concerted effort to make converts in prison. They know that men who are already in many cases notably aggressive and violent, and who have a grievance against society, are perfect fits for a religion that contains teachings that are themselves aggressive and violent, and that sets itself against infidel society. Islamic proselytizers in prisons hope to channel that aggression by sanctifying and thereby justifying it – as Holmes seems to be trying to justify his murders by affecting Islamic piety now.