We should absolutely defend the soldier. Innocent until proven guilty. He will be tried in a court and the evidence will come out.
Part of the problem is the “rules of engagement”. With all of the modern technology available, the soldier (and everyone else) should not have been allowed near the attacker (in case he had a suicide belt) until it was certain (via drones, for example) that he was no longer a threat.
Hypothetically, if someone is not an immediate threat, there is no justification (Halachicly or otherwise) to hurt them.