Pretty much with this build your playing off the poisons and high attack speed that twitch has, and with the high attack speed comes expunges high damage earlier and I normally got 15+ kills and 3 or less deaths and I win the game in generally about 20 as they surrender (RAGE-QUIT)

the runes have been chosen as they enable a good starting damage and attack speed early on in the game enabling you to get fed and get most of your items by level 12.
I prefer plain attack damage runes as you can get kills earlier and none of this 0.033 attack damage per level **** because that doesn't really help.
Speed runes as explained.
magic resistance runes, one because no melee person will get near you and because anyone with good range and damage is a threat as you are remotely squishy but can easily take on anyone 1v1 and late game can easily 1v2 and stand in their base with life steal and AoE passive attacks with Tiamat.
Quintessences more attack speed what's wrong with that??

I use 21-0-9 these as they increase:
Damage and attack speed,
Armour penetration,
Life steal
longer duration for the neutral monster buffs
which you would be going for as the blue buff means you will never be out of mana and the red buff is a poison DoT and makes your attacks slow the enemy

this item set is a little different, as you are indeed a carry as soon as you have purchased the two vampiric scepters after the infinity edge you will dominate, and late game if they have not surrendered and you can keep both bloodthirsters stacked your crits will be hitting over 1000 with over 50% lifesteal AND having 1.5 attack speed(2.something with ambush), with this i have literally sat in the middle of a team fight and killed everything that moved with twitches ULT

the skill order is very specific as it gives you great damage early game to get the starting gold you need to get set off also, I don't raise debilitating poison until level 8 as you want the 70% bonus attack speed and high expunge damage.
I level expunge first as early game when everyone is fighting you can just pile stacks of poison on someone make them waste heal expunge and then make them cry
invisibility is next as after the start game fight you want to be dodging ganks by running into brush and going invisible, saved my *** soo many times, and coming back out then going to base and heal and buy items.
your ultimate is the greatest thing ever think about when you see some1 stood in-front of a tower with low HP use your spec and they WILL die they wont expect you to hit them and the range boost allows you to hit them without the turret killing you.
another situation as said earlier team fights use you ULT then hit 2-5 of them and your HP will not go down :D
and debilitating poison last as late game people will fear you unless they have already surrendered which with high dps and other stuff your slow will be immense at this stage of the game

overall if you don't win a 3v3 game within 25 mins you should really be reading this guide and stay invis for about 10 secs before coming out for full benefit of the attack speed bonus

have fun xD
K/D/A Time
Best Game: 32-4-7 35:34
Worst Game: 6-6-9 16:02 <<<bad lagg

Please have at least 5 games and post your best and worst game as a comment
Another thing don't comment or vote unless you have tried this build