If don't tell her the truth ,she may kill him.?

If you don't tell her the truth, she may kill herself.
If you don't tell her the truth, she might kill herself.
If you don't tell her the truth, she will kill herself.
If you don't tell her the truth, she would kill herself.
以上都正確，只是助動詞用might/would時，語氣較may/will緩和，語意較不那麼肯定（機率的百分比不同）。

可以。

不過問題不應該是"可以"還是"不可以"，而是這樣寫是甚麼意思：[假設是一句劇集/處境的對話]

If you don't tell her the truth, she may kill him.
說話的人(第一人)是跟一個人(第二人)說：「假如不對她(第三人)說實話，她有可能會殺死他(第四人)。」
意思是說話的人(第一人)估計她(第三人)是有很大可能由於第二人不對她(第三人)說真話而殺死第四人的。

If you don't tell her the truth, she would kill him.
說話的人(第一人)是跟一個人(第二人)說：「假如不對她(第三人)說實話，她是可以殺死他(第四人)。」
意思是說話的人(第一人)估計她(第三人)是不是太肯定由於第二人不對她(第三人)說真話而有可能她(第三人)會殺死第四人的。

因此may要比would更深信會發生。

另外還有一句
If you don't tell her the truth, she will kill him.
說話的人(第一人)是跟一個人(第二人)說：「假如不對她(第三人)說實話，她必定會殺死他(第四人)。」
這句的意思比may或肯定假如不說真話她一定會殺死他(第四人)，所以意思是說話的人(第一人)不是估計，而是差不多肯定這因果關係。

In subjunctive condition:-
If clause:-eg:-
If you had not told her the truth,
Main clause:-eg:-
she would have killed him.
may is used to indicate possiblility or probability; as might is used to indicate a future condition;the perfect infinitive might have is used for past time.Probability means quality of being probable,most probable event or outcome..What are the probabilities ? eg:-There is little probability of her succeeding /that she will succeed to kill him.
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eg:-It (may)might,would(will) have killed (him),if you had not told (her the truth).
eg:-If you don't tell her the truth, she (may) kill him.------(might have is used for the past time in subjunctive mood)

用may有徵詢别人的意見，例如May I use your cell phone.在本句無法適用，語意上有問題，所以用would 較貼合句意