Q:

I can be good in a fight

If you are in me  
My name on your breath
Will surely begin me
Yet end soon in death
I'll shock you when high 
I'll please you when low 
But fail to defend me 
To jail you may go

What am I?
The answer is a single word. Please explain how it relates to each line.

A:

The answer is:

Charge

If you are in me 

If you are in charge...

My name on your breath

Shouting “Charge”...

Will surely begin me

Will start a charge...

Yet end soon in death

Which will not last long but will lead to deaths on one side or another.

I'll shock you when high 

If the charge for something is too high, you’ll be unhappy. Alternatively a high electrical charge will provide a shock.

I'll please you when low 

But if it’s lower than expected you’ll be pleased

But fail to defend me 
To jail you may go

If you are in a criminal court on charges and fail to defend them, you may end up in jail.
An explosive charge can be good in a fight.

