なにニヤニヤしてるんだよ。何かいいことあったのか。

What are you smirking at? Did something good happen?

お前が近くにいるのをいいことにベタベタしやがって！！

What the fuck are you up to taking advantage of his proximity to cling to him like a wet T-shirt?!

親が仕事で忙しいのをいいことに子供が友達の家に入り浸ってしまっている。

Since her parents were busy at work all the time, she spent a lot of her time at her friend's house.

どこであれ道路を横切る時に右左を見るというのはいいことだ。

It's a good rule to look both ways before you cross the street anywhere.

これは絶対、情けは人のためならず！いいことしたからご褒美なんだよ！

This is definitely, 'The good you do for others is good you do yourself'! It's a reward because I was helpful!

日本文化に造詣深いことはいいことです。

Being knowledgeable about Japanese culture is a good thing.

君の考えなどはどうだっていいことだ。

Your thoughts are of no significance at all.

あんな調子のいいこと言っておいて、ホント口ばっかりなんだから！

I like the sound of what he says, but it IS just talk, you know.