A Russian-speaker I know gave me this feedback:
якобы - implies that the accusation is questionable
Предположительно - implies that the accusation is reliable (but not necessarily proven)
Also the word киллер is more likely to be used than убийца, according to him.

Speaking of killers, has the movie Американский снайпер been widely viewed in Russia? What has been the reaction of Russians to that movie?