1) Khadr may or may not have actually even thrown the fatal grenade. The only evidence is a heavily coerced confession and US testimony that was changed years after the fact from "some dead middle aged guy threw it" to accusing the live prisoner they just happened to need a good rationale for keeping locked up at the time.

2) Speer had medic training, but was acting in a combat role (and apparently disguised in civilian clothes) at the time. It's a shame he died, but it's not like even if Khadr did kill him, it was a case of selecting a visually identifiable medic to attack.