Make The Monster Despawn When Looked At

I can recommend you something that i've tried and worked for me, but is a bit poor, you need to insert into entity tick this command that would be executed from the entity (but first you will need to make a synched data for the entity(In my case i will call it "Test")):

execute as @p at @s anchored eyes facing entity @e[type=mod_id:entity_id,limit=1,sort=nearest] eyes anchored feet positioned ^ ^ ^1 rotated as @s positioned ^ ^ ^-1 if entity @s[distance=..0.1] run data merge entity @e[type=mod_id:entity_id,limit=1,sort=nearest] {DataTest:1b}

(i recommend you to play with the distance)

This would make the Test synched Data of the entity became true, now inside the tick update you must put a procedure that makes this:

if  entity... Test = true{
   (Here you put your execution)
   (here you must set the Test to false to avoid he loop)
}

Hope this helps you, and sorry if bad english ;)