In my campaigns not all goblins are evil, not all lammasu are good, etc so killing sentient creatures on sight based on race is strongly discouraged. Well, sneaking up and sneak-attacking is a rogue shtick, and I'm asking how I can allow a rogue to play like a rogue?
As SevenSidedDie pointed out, 'sneak attack' is not a literal mechanic in all games, but many games have a rogue or rogue-like archetype, and they tend to be sneaky characters that kill people when they're not paying attention, so I think I know what you're asking.
Just Slow Down
Give them the opportunity to gather more information about NPCs than simply their race. They can then sneak up and kill them if it's warranted.
Just because a rogue likes to sneak attack doesn't mean they need to do it the second they encounter somebody.
If this is actually a problem for you, then you might want to read this: How do I get my PCs to not be a bunch of murderous cretins?