One option could be to check out your local library. Sometimes they have manga collections and might have Romantic Killer. Also, some online subscription services specifically for manga might have it. Just do a quick search to find the right one for you.
It's definitely gained a significant fan base. The interesting characters and engaging plot have contributed to its popularity. Also, the way it combines romance and mystery elements appeals to a wide audience.
There are several options. One could be ComiXology, which often has a wide range of manga available. Another possibility is Webtoons, but it's not guaranteed that 'A Killer Paradox' will be there. Sometimes, specific fan forums or communities might share links to where you can find it, but be cautious as not all sources might be legal or reliable.