我的小狗被车撞死了，我很自责没有给它拴绳，没有看好它。
My puppy got run over, and it's just killing me. I should have had him on a leash and kept a better eye on him.

【Notes】
Got run over: 一个非常口语化且直接的说法，意为“被车撞了”或“被车碾了”。
It's killing me: 英文习语，用来表达“这让我极度痛苦/难过”，非常生动地传达了原文中“自责”和“难过”交织的强烈情绪。
Should have: 意为“本应该做某事（但没做）”，是表达后悔和自责最常用的句式。
To keep a better eye on someone: 英文习语，指“更仔细地照看好某人/某物”，完美对应了“看好它”的含义。