Как по-английски будет звучать "Казнить нельзя, помиловать" . Есть ли эквивалентная фраза?

Я правильно понимаю, что вас интересует не буквальный перевод? Такое явление (когда смысл можно понять двояко) называется амфиболией. Вот примеры из английского языка:

Teenagers shouldn't be allowed to drive. It's getting too dangerous on the streets.
These sentences could be taken to mean the teenagers will be in danger, or that they will cause the danger.

The duke yet lives that Henry shall depose. — Henry VI (1.4.30), by Shakespeare
Amphiboly occurs frequently in poetry, owing to the alteration of the natural order of words for metrical reasons.

Eduardum occidere nolite timere bonum est. — Edward II by Marlowe
According to legend, Isabella of France and Roger Mortimer, 1st Earl of March famously plotted to murder Edward II of England in such a way as not to draw blame on themselves, sending a famous order in Latin which, depending on where the comma was inserted, could mean either "Do not be afraid to kill Edward; it is good" or "Do not kill Edward; it is good to fear":
Fear not to kill the king, 'tis good he die. / Kill not the king, 'tis good to fear the worst. (5.4.8-11)

(Professor to student, on receiving a fifty-page term paper): "I shall waste no time reading it." (Often attributed to Disraeli)
Implies either that the professor avoid all delays to reading, or that he will not misspend his time by reading.

No food is better than our food.
Implies that ours is best, or that ours is so poor that having none is the better choice.

Как видите, по смыслу ближе всего пример про Эдуарда :)

kазнить нельзя помиловать — execute not pardon

there is no love lost between them - самый известный эквивалент

Execution can not be pardoned.

An English professor wrote the words:
"A woman without her man is nothing" on the chalkbboard and asked his students to punctuate it correctly.
All of the males in the class wrote:
"A woman, without her man, is nothing."
All the females in the class wrote:
"A woman: without her, man is nothing."

Еще в тему о заковыристых выражениях:
If the police police police police, who polices the police police? Police police police police police police.
Если полицейская полиция наблюдает за действиями полиции, кто ее тогда контролирует? Ее контролирует тайная полицейская полиция.