If you're going for truth, obviously, no, the person doing the shooting is not yet an assassin. He certainly is attempting to become an assassin.
Definitions are not about trying but about having done so.
The truthfulness we're talking about is not limited to assassin but about any word. To truthfully use a word, all its necessary components must be satisfied.
However, the situation here is more about simplification, the culture of newspapers and range of interpretation. The headline is necessarily a much shortened version of what follows, which may itself also be a simplification of reality. Which is to say that the culture is that
Newspaper headlines aren't always literal and can often be misleading
For the headline to call the person an assassin is simple a telegraphed ('much elided') message; the fuller explanation is that no the person is not exactly an assassin but he certainly intended to be one and it was only minor circumstance (a hairbreadth off in aiming) that missed fulfilling that one component of the definition.
Taken at face value, the headline is wrong (in the literal sense), but it could be judged to be close enough. It is not misleading (he meant to kill the girl), but in a court of law, the outcome is more easily judged than the intention.
This subtle employment of range of meanings, both in allowing leniency ('to try is about the same as to do') or literalness ('I did not have sex with that woman') can be used deliberately to one end or the other as a game of words.