The language is unclear
The intent may have been that you must wait for your Action to complete before utilizing your Bonus Action.
Jeremy Crawford had previously tweeted that Bonus Action timing was completely up to the player regardless of the trigger requirement. While this gave a lot of freedom of choice, it wasn't really fully in tune with the written rules on Bonus Action timing (emphasis mine).
You choose when to take a bonus action during your turn, unless the bonus action's timing is specified...
However, that tweet has been updated after realizing his mistake.
Clarification about bonus actions: if a feature says you can do X as a bonus action if you do Y, you must do Y before you can do X. For Shield Master, that means the bonus action must come after the Attack action. You decide when it happens afterward that turn.
He further clarified with this tweet
No general rule allows you to insert a bonus action between attacks in a single action. You can interrupt a multiple-attack action with a bonus action/reaction only if the trigger of the bonus action/reaction is an attack, rather than the action.
But that clarification was...wait for it...further clarified again:
"My tweet below was addressing bonus actions and reactions that have triggers. A bonus action that has no trigger—such as Cunning Action and the misty step spell—can take place whenever you want on your turn (PH, 189)
The RAW and the Crawford
It seems clear that Crawford's intent for Bonus Action triggers is that there is a requirement for the triggering Action to fully complete before you can take the Bonus Action. If you go by this, then you are not able to take your Bonus Action inside of (interrupting) your Attack Action.
A question of wording
However, as stated above, GWM is not triggered by the completion of an Action, but by the killing of or reducing an enemy to 0 HP. Given that the trigger is not based on the Attack Action but on the result of a hit combined with the rule on choosing when to take a bonus action during your turn, then it seems possible to interrupt your Action.
Which way to go?
I understand what Crawford is saying in that Actions are complete events that can not be interrupted...except by movement. His clarification that you can't interrupt one action (Attack) with another (Bonus Action) makes some sense, but given the precedent of allowing movement inside of an Action and that the trigger for GWM is not the Action but an event, a GM could rule at their table to allow it.
Crawford's intent here is understandable, but it still doesn't make 100% logical sense given that you can already interrupt your action with movement and/or reactions.
At my table, I generally let my players shift and shuffle actions as long as they aren't actually limited by a clear mechanic. In this case, it makes more than good sense to allow the player this and I haven't seen any issues with it at my tables.
Yes, it would trigger the bonus action and it would be also a critical hit.
The feat Great Weapon Master calls for a critical hit, not a natural 20. This is important because Assassinate is not the only ability that modifies the critical range, e.g. Improved Critical from the Fighter also modifies the critical range. Also, this bonus attack would be a critical hit. It may seems OP but bear in mind that melee rogues usually wield two weapons for the benefit of Two-weapon fighting. Furthermore, since you only have one bonus action both features are incompatible.
Improved Critical: Your weapon attacks score a critical hit on a roll of 19 or 20.
Best Answer
Yes, you can move before taking the bonus action attack
No timing is specified
The phrasing here says nothing about the precise timing of the bonus action. It can be simply read as saying that after the condition is met (critical hit or reducing to 0 hp) that you are granted the ability to make a bonus action attack. It never says the bonus action must be taken "immediately" or anything along those lines so the timing of the bonus action attack is not restricted at all.
The rules for when you can take a bonus action say that if the timing is not specified, then you can choose when to take it. Thus, there is no reason why you would not be able to choose to move before taking the bonus action attack.
The rules explicitly allow moving between attacks
Even within a single Attack Action, the rules allow one to move between attacks. It absolutely makes sense that they would allow it between an Attack and a bonus action attack for example. After all, even if the bonus action isn't considered to be part of the Attack action (or whatever triggered the GWM bonus attack condition), it is indeed a weapon attack and thus moving between them seems like an incredibly reasonable interpretation.
Jeremy Crawford has agreed this is the intended interpretation
Jeremy Crawford has clarified this exact issue via Twitter and agrees with the interpretation above: