You have explicitly imposed the constraint

without invoking meta knowledge

but your question is implicitly meta. I think you find this situation difficult because it is a meta problem, but you are pretending that meta is a bad thing.

Sure, if "my guy" is not suicidally brave, running away and abandoning the fallen would be in character. So why not do that? Because that would lead to savable characters dying, which would be not fun for your fellow players, and could shut down the story of "who sent these assassins?", which would be not fun for the GM (who presumable has put some work into setting up that story). Being concerned about your fellow players (including the GM) having fun is a good thing, and it is a meta concern.

But you are not alone in addressing the meta concern that everyone should have fun. Everyone at the table is responsible for that. So don't act like it is only your concern. Clearly request that there is a time-out to discuss the issue. In particular, point out to the GM that how the encounter has played out so far has created a problem that needs to be fixed, and it needs to be mostly fixed by the GM. You should point out that your character suicidally attempting to rescue the other characters would be out of character and unlikely to be successful, leading to a (no fun) TPK, but that running away would also be no fun for most players. Everyone at the table should be involved in creating an agreed fiction that fixes this problem and that everyone is happy with.

I can think of several possible adjustments.

• The GM retcons the combat so far to describe their success as due to luck, retcons their abilities to be weak enough for your character to be able to defeat them, and have your character.
• The GM provides the meta information that the downed character will not be executed on the spot, but will be captured, so the next phase of the adventure will be a combined rescue mission/prison break out. (my preference).
• The cavalry arrives, driving off your attackers.

You have explicitly imposed the constraint

without invoking meta knowledge

but your question is implicitly meta. I think you find this situation difficult because it is a meta problem, but you are pretending that meta is a bad thing.

Sure, if "my guy" is not suicidally brave, running away and abandoning the fallen would be in character. So why not do that? Because that would lead to savable characters dying, which would be not fun for your fellow players, and could shut down the story of "who sent these assassins?", which would be not fun for the GM (who presumable has put some work into setting up that story). Being concerned about your fellow players (including the GM) having fun is a good thing, and it is a meta concern.

But you are not alone in addressing the meta concern that everyone should have fun. Everyone at the table is responsible for that. So don't act like it is only your concern. Clearly request that there is a time-out to discuss the issue. In particular, point out to the GM that how the encounter has played out so far has created a problem that needs to be fixed, and it needs to be mostly fixed by the GM. You should point out that your character suicidally attempting to rescue the other characters would be out of character and unlikely to be successful, leading to a (no fun) TPK, but that running away would also be no fun for most players. Everyone at the table should be involved in creating an agreed fiction that fixes this problem and that everyone is happy with.

I can think of several possible adjustments.

• The GM retcons the combat so far to describe their success as due to luck, retcons their abilities to be weak enough for your character to be able to defeat them, and have your character decide the risk fighting.
• The GM provides the meta information that the downed character will not be executed on the spot, but will be captured, so the next phase of the adventure will be a combined rescue mission/prison break out. (my preference).
• The cavalry arrives, driving off your attackers.