is it because (before anything happen) Mr.X is already a thief "metaphysically"?
If it was intentional, then Yes in an immediate and conscious sense, but otherwise No in the more important long-term or "identity" sense, as of when he hadn't intended to do it.
Important to note:
People change. Things can change too, but they lack intent. Therefore identities are capable of transformation.
Action precedes transformation of status and/or identity, but status and identity also influence action.
Let's borrow a concept from computer programming - Duck Typing.
Loosely, Duck Typing says, "If it does Thing A, It must be a Thing-A-Doer", or concretely, "If it sins, it's a sinner". It is the simple convention of upgrading a verb or attribute to a noun.
By this convention, Action precedes the transformation of Being.
If you have lots of money, you are rich. You do not have money as a consequence of being rich. The action drives the state.
We can also borrow from Spanish, which has two main verbs for "to be": ser and estar. Use ser to indicate identity, profession, or other long-term conditions, such as, "My name is...", "I am American", etc. Use estar to indicate temporary conditions or statuses, such as "I am sick", or "he is over there".
Transient statuses do not remain, and the differences between status and identity is that the activities associated with the former will cease once the condition has ended, while the latter persists and does not recur or end with changing circumstances.
Then we have a simple cycle:
- Doing leads to Being.
- Being then leads to more Doing.
That is, if it quacks, it's a duck, and the more ducklike it becomes, the more it quacks. The relationship between state and transformation is self-reinforcing.
Another analogy, this time from physics:
An accelerating magnetic field induces an electrical current.
An accelerating electrical current can also induce a change in the magnetic field.
By flowing electricity in a loop around an iron rod, the rod becomes temporarily magnetized. After electricity has flowed around it, it "is" a magnet. Not only is it a magnet, but it now induces electrical flow in nearby electrical conductors whenever it accelerates. This shows that the completed cycle of "do->be->do" is physically valid.
If you want some additional nuance for human actors above the two-step circuit above, thought precedes action, so yes, a person becomes a thief by intent even before the completed action. However, without evidence either of intent or completed action, the identity or status designation is not apparently deserved or applicable.
So an expanded cycle for an intentional actor introduces another (you might say metaphysical) stage:
- Thinking leads to Doing.
- Persistent Doing leads to Becoming.
- Being influences how we think,
- How we think determines how we act.
Interestingly, beings who think and act have the power to transform their identities over time, which is both a blessing and a warning regarding our behaviors.
All of which confirms Jesus's saying "As a man thinketh in his heart, so is he".
Due to the real nature of identity and persistent properties, any phenomenon that has not been fully reversed can safely be assumed to be part of a longer-term identity. In your example, it cannot be refuted that the man is a thief if he has stolen and has not repented and made full restitution and compliance with the requirements of the law. However, if the action can be demonstrated to be transient (he has fully repented, made full restitution, complied with the full demands of the law, and does not do it anymore, and his intentions are remitted and repaired through Divine grace), then one cannot make the case that he is a thief any longer.