I don’t know if I’d say it’s “obvious” he was murdered without more details. Yes, it’s incredibly likely - probably the most likely cause. That being said, people that appear healthy can and do die suddenly quite regularly - heart attack, stroke, aneurysm, etc. can occur at any age and even with healthy people, often with very little visible advance warning. He’d been poisoned in the past - who knows what damage that may have left. Though I would argue that would still count as murder, even if as a delayed effect.

Still, yeah, he was probably murdered.