Meaing of phrase 切り裂きジャックだから怪しい、なら分かるけど

[Yoshie]

I can't completely understand the purpose of the last sentence (in bold) in the following passage. What does the speaker wants to say with it? Maybe something like "Because it's Jack the Ripper it's dubious (if he belongs to ABYYS), then I can understand."

意味が分からないものの、同意する。
僕があったのはあれで二度目だけど、姿や特徴から察するに、あれが切り裂きジャックであるのはほぼ間違いが無い。
黒塚「切り裂きジャックなら、やっぱり彼はABYSSではないわ」
話し手「切り裂きジャックなら・・・・・・？」
黒塚さんの言っている意味が分からない。
切り裂きジャックだから怪しい、なら分かるけど。

Context: The speaker and 黒塚 just met with the person who is called 切り裂きジャック. He ran away from them. While speaker thought that he belongs to ABYSS group which both of them were looking for, 黒塚 is saying him that if that person is 切り裂きジャック then he is not belong to ABYSS, she is saying 'if' because she just seen him for the first time.

 

[Majestic]

The speaker didn't understand Kurozuka's supposition that "If that person were indeed Jack the Ripper, then (logically speaking) Jack the Ripper wouldn't be associated with ABYSS.
The speaker could understand if Kurozuka had simply said "That person is Jack the Ripper, so naturally he's suspicious". But for some reason the conclusion about the non-connection with ABYSS was lost on the speaker.

In other words, Kurozuka was putting together some clues about Jack the Ripper and ABYSS that the speaker wasn't quite aware of.

Does that make sense?

 

[Yoshie]

Oh, so it was about Jack himself rather then him belonging to Abyss.
It makes, thanks.

Though, I'm not sure how do you understand that 'suspicious' refers to him, not to his connection with Abyss?

 

[Toritoribe]

The speaker's thought;
Jack the Ripper has something to do with ABYYS.

Korozuka's thought;
Jack the Ripper has nothing to do with ABYYS.
He is Jack the Ripper, therefore he has nothing to do with ABYYS.

The speaker can't understand why Korozuka thinks Jack the Ripper has nothing to do with ABYYS.
怪しい means "he belongs to ABYSS" there.