How to grant permission for something in Japanese


I know that させてくれないか is _won't you let me?_  
But how do I say _allow_? させてあげる _I'll let You!_.  
I want it to be "something + the auxiliary _ageru_ ".

Btw.  
Why is 借りしてもいい right but not 借りてもいい ?!?!?

I want it like this:

> (部下に) よし。そいつを殺させてあげる。はははは (悪声)

I want to grant permission without a request having been made in the first
place, and I want to be firm when I grant the permission.




～させてあげる is perfectly correct, but in your context (i.e., to sound firm,
bossy...) I think you could use やる instead of あげる, as in:

> よし。そいつを殺 **させてやる** 。はははは

* * *

And, as stated in the comment by @Yuuichi, 借り **し** てもいい is not correct. You'd
say 借りてもいい, or **お** 借り **し** てもいい in 謙譲語/humble language. （Examples of the
humble 「お + 連用形 (continuative form) + する」 form: 借りる ⇒ お借りする, 届ける ⇒ お届けする, うかがう
⇒ おうかがいする）

