I tend to think that it would be unrealistic to expect Sickles to not honestly believe his corps - and, consequently, the Union army - to be in danger given the events that took place leading up to his decision.
He was correct that a major attack was coming in his sector.
When he asked Meade for clarification, Meade fobbed him off with Hunt - and then when Hunt concurred with Sickles, Meade did not go over and look himself.
Sickles had also faced a situation just recently in which his corps was bombarded from dominating terrain.
What we do not know however is whether Sickles' corps was actually in danger of being defeated if it had retained the position it had been assigned. I think, though, that if 3rd Corps had been able to defend itself (if attacked in that position) then we would probably expect the reinforcements to have been able to defend themselves against Longstreet (in that position) without much trouble with no greater strength needed than that of the original 3rd Corps. Instead the attack caused massive havoc even with an entire Confederate division veering off to attack the Round Tops.
This means I think there is a reasonable expectation that Sickles would not have been able to resist the attack that was actually coming, in the position he was supposed to be in. That doesn't mean it's impossible, it means we can't say he was almost certain to hold (say).
Now, given the above, the appropriate thing for Sickles to do - given what he knew and could believe - would be to strongly argue that his corps should take up an advanced line. What is at question is whether it would then be justified for him to take an action (i.e. taking an advanced line) when it is evident that the army commander is ignoring him.
Colouring this decision is that Meade is extremely new to the job, of course.
I think Sickles' movement is probably justifiable in the situation, if only in the Prussian spirit of "after the battle is over, my lord may have my head, but in the meantime allow me to exercise it in his service".