Can an opponent kill Undercity Informer before you can sacrifice it?
Normally, your opponent will not be able to stop you with a kill spell.
When a spell resolves; the active player gets priority:

116.3b The active player receives priority after a spell or ability (other than a mana ability) resolves.

Normally if you are casting a creature spell, that means you are the active player; as you can normally only cast creature spells on your turn. Because you have priority, you can activate Undercity Informer’s ability; before your opponent had any chance to act. You pay the costs when you activate the ability, which includes sacrificing a creature. Whether you sacrifice Undercity Informer or a different creature; the ability is activated and on the stack. Only then does your opponent get priority.
Also, even if you do not choose to activate the ability the first chance you get; if your opponent casts a kill spell, then you can respond to that by activating the ability at that point.