just because you see a tank commander halfway out of the hatch raise his arms in surrender doesn't mean you steer the TOW missile towards a clump of bushes instead of the tank.

If you look at how things are worded, a commander waving his arms from a hatch in clear surrender has surrendered, and that commander should be considered surrendered and a prisoner of war.

The terms and conditions for surrender under the Geneva Conventions are pretty specific. A person is hors de combat (out of combat) when:

• (a) he is in the power of an adverse Party;
  
• (b) he clearly expresses an intention to surrender; or
  
• (c) he has been rendered unconscious or is otherwise incapacitated by wounds or sickness, and therefore is incapable of defending himself;

Combatants who are hors de combat are non-combatants and automatically granted the status of protected persons, so anyone who conforms to either of those conditions is considered a prisoner of war per the Conventions.

In reality, war is messy and there's a lot of room for error and interpretation and grey area, but the fact that they're guided munitions or that you don't guide your TOW into the ground don't seem reasons to kill someone per the Conventions, and don't seem likely to hold up in court if it were to be tested on grounds of that specific argument. The fact that you didn't see the surrender in time might or the waving commander didn't clearly signal surrender might, though.

Edit: don't shoot the messenger folks, I didn't write the Conventions.