should all new clothes be wash before user wears it ?
This includes all tactical pants, shirts etc.

this sounds like a dumb question. pardon me.

I don't know about "should" be washed, but I generally wash new clothing to clean out the sizing chemicals and to soften them (and get rid of packaging creases). However, I don't wash new socks, because I LOVE that feeling.

If the clothing is used, then my vote is "Yes."

For pants, I have to in order to get the initial shrinkage out of the way before hemming.

I always wash new underwear and workout gear. Other stuff, it's a judgment call.

I wouldn't bother with washing. before wearing unless instructions with the clothing specifically called for the practice

I bought the new tactica 5.11 pants. It is made in Vietnam instead of China

As a rule, it's best that new clothes are washed if they will have a lot of skin contact. Some of the dye residues and other chemicals are probably not all that good for you if absorbed, or may cause nasty skin irritation. And keep in mind that offshore manufacturing is a wild west in terms of chemical safety.

The exception to the rule: guys mostly don't bother to wash stuff. Whether this is good or bad is up to you.

I normally wash new clothing before use in case it contains harmfull chemicals, or has been packed or handled by someone suffering from infectious disease.
And I wonder if "new" clothing has in fact been tried on, and perhaps found to be the wrong size and returned ?

The risks are small, and I would not worry in an emergency, but whilst times are normal it seems a good idea.

Virtually all clothing sold in the UK is marked "wash before wear"

I generally bathe before donning new clothing, figuring that it is probably cleaner than I am. YMMV.