First, a bit of context: there is a knight who grew up with the princess, and they were very close; in the end they killed each other (which is said in the prelude, and then the story kinda goes back).
The princess then is shown as ruthless, executing people deemed as rebels and their families with cruelty, like first killing the young son just to make the parents suffer.
Meanwhile the knight is shown as blind and is treated as a rebel by her former comrades.
In the scene, the knight is figthing with her former teacher, and the point of view is another character.
This is the sentence I'm not sure about:
I read 陛下を残しては死ねないのです in two possible ways:
"If Her Majesty is still alive, I can't die" (with this meaning of ては), and since the knight is a rebel and the princess a tyrant, it'd make sense if the first wanted to stop the second; but I don't understand how それが「陛下を残しては」という今の台詞と結びつかない follows: I think it means "That [the cruel executions and loud laughters] doesn't fit the sentence 'If Her Majesty is still alive'", which doesn't really follows.
"I can't die leaving Her Majesty behind", which - if 残す as "to leave behind" as a meaning of abandoning - would make sense with the following part, since the point of view character wouldn't understand why the girl would be reluctant to abandon such a tyrant; in this case, ては sounds to me like an "and" ("I can't leave her behind and die"), but I'm not sure if 残す can have that implication, nor if it's right to read ては that way.
Despite the princess (now ruler) being a girl, リーベルヴァイン王 refers to her, since the point of view character saw her doing those executions, and thought about her "あれが、リーベルヴァイン王".
I think the knight's sentence should make sense given the background (knight and princess know each other since they were child, and they were close), but the narrator - not knowing their background - sees a mismatch beetwen the tyrant she knows and what the knight said; I think the second meaning is the right one ("I can't die leaving Her Majesty behind"), but I'm not sure and I am unsure about 残す and ては as said above.
The quote is from 僕の愛したジークフリーデ vol. 1, by Matsuyama Takeshi.