Because of too much sorrow with 〜あまり
Grammar Explanation: Because of too much with 〜あまり

An image of Queen Serenity tells Sailormoon and her friends about the day the Moon Kingdom was attacked, and how Prince Endymion of Earth came to its defense.
- クイン・セレニティ:
- 「ーーー遅かった」
- “...it was too late.”
- 「彼
( はあなたを守( り倒( れ」- “To protect you, he fell.”
- 「悲
( しみのあまりあなたも自害( を……」- “Because your sorrow was too much, you took your own life.”
- “...it was too late.”
Key Points
悲
( しみのあまり gives an overwhelming cause〜のあまり means an emotion or state is so intense that it leads to what follows.
Here the cause is not ordinary sadness, but sorrow great enough to drive the tragic action.
The noun before あまり takes の
Because 悲
( しみ is a noun, it links to あまり with の.With verbs or adjectives, the form before あまり can look different, but this scene uses the noun pattern.
自害を… is deliberately unfinished
自害
( means taking one’s own life, and the omitted verb is understood from context.The trailing sentence keeps the retelling solemn and avoids stating the act too bluntly.