Cause all bots are bugged. They all run in your gas, after fling are 1 second staying desorientated, especially under turret. They always turret dive. They always focus those champions with low lives.

So you will not be ever focused by them in mid or late game, you will get your first 3 kills during levels 2-6 by turret fling and gassing enemy. In late game you will have 5000 lives, which makes you pretty unkillable even if you stay alone. Your ability power will be something around 750 - your gas will deal 250 magic damage over second. Your fling 1000. With ulti, ghost and Mega Adhesive, nothing escapes from you.

Always harras the stupid bots with gas, escape with cleanse from stuns or ugly slows and taunts. Let them come to turret, fling them under it and run behind them.

They will follow you even if you are faster, so you can safely slowly kill them with gas, jus if you are faster.

You should take bottom lane, where I always met 2 meelee champions (Renekton, Cho, Shen...). It is also better lane for you, because Soraka takes top, there is much harder to get kills for singed, her heal seems to have no cooldown. Don´t forget that Singed is strongest vs. slow meelee champions.

If you cant get first blood, use your potions to recover at least half of your health and get enough mana for fling, let them push to your turret and then get your first blood at second level by turret-fling.

This build gives you a lot of lives to make bots focusing another champions, a lot of ability power, for easy getting kills at late game. Instead of Rabadon´s Deathcap, you could get also Rylai´s scepter for slow and more lives.

I don´t believe that this build would work in normal game, except of those full of noobs, actually i never tested it there...
I think singed works well as an mana offtank in 5v5, you can see my build for one, if you wish.

This build uses bugs and holes in bots AI, so it will not probably work after some patches from Riot. Every Singed is overpowered vs. bots, this one is created as the best killer. Have fun and make allies call you "Legendary Stealer"!