〜せいで

雨(あめ)のせいで遅(おく)れました。

Ame no sei de okuremashita.

I was late because of the rain.

せいで is for bad outcomes only, and it assigns blame

私(わたし)のせいで、みんなに迷惑(めいわく)をかけました。

Watashi no sei de, minna ni meiwaku o kakemashita.

It is my fault that I put everyone out.

Pointed at yourself it becomes an apology

結果(けっか)が良(よ)ければ前者(ぜんしゃ)、悪(わる)ければ後者(こうしゃ)です。

Kekka ga yokereba zensha, warukereba kōsha desu.

Good outcome takes the first, bad takes the second.

This pair is the most confused at N3, and the test is simply: was the outcome good