Sentence check

[Lightna]

I've been writing a story as of late and I need to check if I have my Na'vi right.

"Slä moe tam kempe moe zene tam."

"Oe tam kempe Oe zene. Oe tam ke new ne, slä Oe zene."

"Lonu kempe nga hasey. Txoa za'u mì..."

If there is a Na'vi word(s) for "the end," please mention it.

Irayo and Eywa ngahu.

[Carborundum]

Slä moe tam kempe moe zene tam

But the.two.of.us do what.action the.two.of.us must do
I'm guessing you were going for 'but we do what we must', but you're quite a bit off the mark, unfortunately.
A few things:

Tam = do, as in 'that will do', not as in 'I will do this'.
-Pe- is a question marker. It can ONLY be used in questions, not statements.

Second sentence is much the same. Don't worry about it though, this is complex stuff. I'd try:

Slä moe fikem si a moe zene
But the.two.of.us this.action do which the.two.of.us must

This is a potentially incorrect usage of si. I'm not sure it can be used with a demonstrative. It's the best I've got however.

Oe fikem si a oe zene. Oe ke new, slä oe zene
[same as above]. I not want but I must

I'm omitting fikem si in the second part of this sentence because I think it would be understood.

Lonu kempe nga hasey. Txoa za'u mì...

Release what.action you done. Forgiveness come in...
I give up. I have no idea what you were going for with this.

[Lightna]

Slä moe tam kempe moe zene tam.
should be:
But we do what we must do.

Oe tam kempe Oe zene. Oe tam ke new ne, slä Oe zene.
should be:
I do what I must. I do not want to, but I must.

Lonu kempe nga hasey. Txoa za'u mì...
should be:
Let go what you done. Forgiveness will come...

I wanted someone to check over it because I'm unsure if I got it right.

Irayo for your srung tsmukan Carborundum.