It is a phrase and it means “bad character”. It is used when someone/something is nasty. You can also use its adjective form ”たちの悪い”.
I’ve been under the weather for a few days. My fever has gone down but my cough isn't getting better. It's a bad cold.
ここ数日体調が悪い。熱は下がったけど、咳が止まらない。たちの悪い風邪だ。
ここ数日体調が悪い。熱は下がったけど、咳が止まらない。たちの悪い風邪だ。
Ken is complaining about Leo's behavior.
- Ken : I asked Meg on a date but she blew me off.
メグを(デートに)誘ったけど、断られたよ。 - Leo : Man, sorry to hear that.
そりゃ、残念。 - Ken : Did you know Meg has a boyfriend?
彼氏がいるって、知っていた？ - Leo : Ah, yeah. I often see them together.
あっ、ああ。よく一緒にいるのを見るから。 - Ken : Why didn't you tell me even though you knew I liked her? You're such an asshole.
俺が彼女を気に入っている(or好き)って知っていて、なんで教えてくれないんだよ？お前、たち(が)悪いなー。 - Leo : Sorry. I didn't know you were serious.
ごめん。マジだとは思わなかった。
Meg was involved in a traffic accident. She is talking to Amy about it at their office.
- Meg : I was late today. I got into a traffic accident, and the police thought I caused it. The actual culprit had already left when the police came.
今日遅刻しちゃった。交通事故に巻き込まれて、加害者(*1)だと思われちゃった。警察が来た時には、すでに犯人がいなかったから。 - Amy : Did he escape?
犯人、逃げたの？ - Meg : Yes. The police said that he had caused such accidents several times before on purpose. It seems speeding relieves his stress.
うん。警察曰く(*2)、似たような事件を数回引き起こしている(*3)らしい。スピードを出して、ストレスを発散しているらしいよ。 - Amy : OMG. That is a nasty incident.
最悪。たちの悪い事件に巻き込まれたね。 - Meg : What makes things even worse is that he doesn't care that he is bothering other people.
他人に迷惑をかけている事に気付いていない事が、余計(*4)たちが悪い。
*1 加害者<-->被害者
*2 曰く(いわく) is another expression of "が言うには".
*3 事件を(引き)起こす means to cause an incident.
*4 余計(よけい) means "even more" in the context.