why did i need to leave 1 neutral after i steal?, it takes more time to respawn?

It won't respawn at all and doesn't reset the timer for it to respawn until they kill the one that you left. It puts their jungle behind by several minutes if you get a buff, or a minute or two if you get something small. Over time this can be significant, and adds up if it's repeated.

Packs only respawn after all have been killed. Kill the big one, the others are more than likely a waste of your time.

yes it does, blue for example has a respawn time of 5 minutes. this timer starts after all
minions of the camp have been killed.

But you still want to clear it full, so you infact know when its going to respawn and you can steal it.
Never understood people leaving one behind.

You leave the neutral so that when their jungle comes around they get less exp and gold from it and have to wait longer for it to respawn. I personally am in favor of killing all the creeps and then having the timer for the camp.

they do not respawn till they are killed. so the other jungler has to kill and waste there time. and sometimes it can respawn when they go to it b4 they go to it and ur steal is effectively u killing ur own camp.