You're getting warmer. Remember that the Anglo-American criminal law system (and previous systems as well) are not only about rehabilitation. Punishment (i.e., "justice"), deterrence, and restitution are also legitimate objectives of the criminal law.
In addition, negligence can properly be addressed by criminal punishment as well as by civil penalties. Let's look at a few categories of intent:
1. Some crimes require a very high degree of intent -- "specific intent" (e.g., first-degree murder).
2. Some crimes require a high degree that is not quite that high -- "intent" (e.g., felony murder).
3. Some crimes can be committed with even less intent -- "recklesness" or "criminal negligence."
4. Finally, some crimes can be committed even without any proof of intent at all -- "strict culpability" (e.g., a speeding ticket).
Your argument that this killer was not guilty of level-1 or level-2 intent doesn't mean that he might still not be guilty of level-3 criminal intent. It seems very likely that he was, in fact, guilty at about that level. It's the same level of intent that can get a mother locked up for frying her forgotten baby in a locked car, or get a frat boy locked up for playing russian roulette with the gun pointed at his buddy.
Bottom line: your suggestions about restitution and community service are good ones. But please don't downplay the severity of this crime or try to excuse the killer just because he wasn't a cold-blooded murderer. If he did the crime (which IMO he clearly did), he should do the time.