Since he/she is someone who does such a terrible thing, it's a good thing that he/she didn't become a parent to begin with.
ご質問ありがとうございます。
"Since he/she is someone who does such a terrible thing,"＝「彼・彼女はこんなひどいことをする人だから」
"it's a good thing that he/she didn't become a parent to begin with."＝「最初から親に彼・彼女がならなくて良かったよ。」
ご参考になると幸いです。
- "It's a good thing they never became a parent."
"it's a good thing ~" 「〜でよかった」"It's good ~" も言えます。
"they never became ~" 「最初から〜ならなくて」
"a parent" 「親」
- "It's good that he never became a father" 「彼が最初からお父さんにならなくてよかった」
- "It's good that she never became a mother." 「彼女が最初からお母さんにならなくてよかった」
アメリカ合衆国 
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