So. i want to run 7 Towers. and in the same time. i want to be able to play diablo 3.

the problem is. if i use tower. i need to run them all with Decrease the background performance, and if i do that, and i run 5+ bots. my Cpu gets pretty much loaded. by 32% each L2.exe,
But, if i tick it off, they are slow cast every 5 sec only.
i used to use L2net. i did not have that problem there.
is there anything to do here. or is it just because l2tower use injection?

well i don't believe you didn't had problems like that on L2Net.
Why?
because when you turn off "Decrease the background performance" in game than from default it start using CPU like active window. If you inject or not inject L2T it will be always like 30%+ CPU use if you have i3+ CPU, on core 2 duo it is like 50% CPU use.
Buy NASA PC than you will be able do what you need (witch is ruing 7 active L2 windows). Tower also increase CPU use, but it will be felt by user after some time of play and it also depends on plugins/scripts you use.