If you're unable to kill anything with auto attacks, you're underequipped. Auto attacking out of stealth gives the auto crit and the 50% dmg bonus for the entire animation chain unless you are interrupted. That is 7 crits for 700% weapon damage that include rune damage and cull the herd passive whereas skills do not benefit from either. You won't get aggro if you do it properly because you generate no threat when you're flanking.
I don't know about you guys but my assassin can kill every non elite with just 2 basic attacks out of stealth on threatening, 4 sometimes 5 attacks on perilous which is still less than 1 chain. I'm doing this with 2 stupid white ~300 DPS daggers with +16 runes in them, give me a good blue or purple dagger set and I'll wipe the floor with regular mobs on perilous pretty damn fast too.
Ideally you're not only using your skills because killing something every 8 seconds makes you useless. Your basic attacks get 50% more dmg from stealth 100% more from cull, they get auto crits and your flank damage bonus whatever that's sitting at. Twin fangs is 1000% and an basic attack chain is 700% basic attacks are hardly "wrong". Oh and the cull the herd passive that only works on basic attacks gives 100% more damage, not 50%. So technically its 7x200% weapon damage crits if you're hitting a loner.
http://i.imgur.com/VcpEATm.jpg