Ok, so, when I first saw the scene involving the Monarch, I wondered for a while why did Salazar tell Henry that he always leaves on man alive, when he clearly didn’t even know Henry – or even that pirate – were below deck before Henry tripped over that bucket? Did he genuinely think everyone had been killed? If so, would that mean that he does not always leave one man alive, and that he only leaves someone alive if that someone is stupidly lucky enough to not be killed? If so, it would mean that there would have been instances where he has killed entire pirate crews and British crews without even leaving ONE survivor, so he probably leaves the whole “Leave one man alive” thing up to chance; if you’re lucky enough to survive, or stay alive for as long as possible, you get to live. I also think he leaves it to the person themselves; I don’t think he kills people who show him respect, or who are honest with him, and here is what I mean by that: In this scene, as he is walking up to Captain Toms, we assume that he is only there to kill Toms, and sure, he did eventually, but what if Toms was the one he was initially going to leave alive? After all, Toms is the only one left alive apart from Henry and the random pirate in the brig, but Salazar did not know of their presence until later.
Unfortunately, Toms makes the situation worse for himself by saying “What are you?”
Now, we all know how proud Mando is, so being addressed as a “What” rather than “Who”, is understandably going to irk the man, especially when you take into account that he does not like to be reminded of his current physical appearance. The second Toms says this, Salazar has him by the throat, responds with “Death”, and breaks his neck, proving that Salazar’s short temper also plays a part in whether he decides to let you live or not … Basically, be careful what you say to him, cos you WILL regret it. Now, another thing, when Henry tripped over the bucket, and Salazar realised there was another survivor, he smiled. I think he was actually happy in a way that someone was still alive, not because he was concerned for them, but because here was another chance of him being able to send a message. Henry, unlike Toms, was more composed, did not scream or yell – which Mando also does not like – and spoke to Salazar relatively respectfully, answered his questions and did not make remarks about his appearance. This is why I believe he left Henry alive, not just because Henry was looking for Jack, though I’m sure Salazar found that very convenient.