If your boy is under 12 weeks old then he can be put with another boy that is also under 12 weeks old.
If he is over 12 weeks old then he needs to be gradually introed with another rat.
http://www.ratshackforum.com/forum/showthread.php?t=32368
You wrote that he repeatedly attacked a young baby without drawing blood and then the baby died. If the baby wasn't injured as you thought, then I would assume that the baby was very ill.
This would mean that your boy may have been exposed to illness, so you will need to keep a close eye on him.
http://ratguide.com/health/basics/basic_health_check.php
http://ratguide.com/health/basics/advanced_health_check.php
They are suggesting that you adopt a boy from the same place to reduce the chances that the new boy will be ill from something your boy has not already been exposed.
I would suggest that you adopt from a good rat rescue as they will be able to help you with intros, refer you to a good rat vet, and the new rat should have been quarantined for any possible illness.
I am very concerned that the young baby died.
Also, rats are like cats .... they are not going to "behave", especially not babies.
While he will become friendlier once he gets a friend, gets used to you, and is socialized (he came from a horrible abusive environment where he likely never was handled gently by a person), he will want to escape, explore and run around
If you read the various sections of this forum, it will give you an idea of what sorts of things rat need
good luck