Could you turn a creature into say a camel and use disintegrate or immolation to instant kill it without it reverting back since they are written similar to PWK?

Changed to camel bc violent fungus isnt a beast for polymorph but works for true polymorph.

RAI: No.
RAW: Maybe.

The wording states that at 0 hit points they immediately revent and excess damage is carried over and if the damage doesn't reduce the original/true form to 0 hit points it won't be knocked out.

However, the Distingrate and similar abilities do state when the target is reduced to 0 hp - which technically they did do - the target dies.

So, really, it's up to the DM to decide which applies first.

This is more of a designers messing up, though, as they did clarify both in sage advice (albeit referring to wild shape rather than polymorph but they both change hit points the same way) and other places the intent is for reversions to always occur first, making the HP effect of Polymorph or Wildshape work more like temporary hit points, except they didn't want this to conflict with actual temporary hit points. The intent from the designers was that reversion at 0 hp back to original form HP occurs before any ' when damage reduces target to 0 HP' trigger effects. They did not word it as clearly as they should have.

PWK, however, is different because it doesn't do damage. Your reversion to original form only triggers when you "drop to 0 HP" due to taking damage. PWK doesn't do damage, it just kills you. And since Polymorph and Wild Shape replace your current hit points with that of the chosen form's, then the combo works. Technically you do revert back to original form, but you're still dead.

Cyb3rm1nd did an excellent job explaining this^

Thank you i was wondering because how can it revert back if it ash.but if it reverts before turning to ash it makes sense.i knew about PWK working was just wondering how it worked with these 2.