Aside from the debate whether it sounds better to say sneaked or snuck (more a question of dialect rather than grammar), they're fine.
Some writers might think these overly wordy though, as "stabbed from behind" pretty much includes the concept that the murderer approached the victim from behind, and caught her unawares.
The man stabbed her from behind, and vanished before anyone could get a good look at his face.
Naturally this depends on the situation. In a police report, for example, it might be important to explicitly say the murderer made no attempt at stealth:
According to witnesses, the suspect walked up to the woman from behind, without any indication he intended violence. He then proceeded to stab her one time in the chest before running away.