I don't dislike the idea, but I think you'd need to change the behavior of c4 for just this round to have a minimum timer of 20 seconds or so, it should only be able to happen on certain maps, and there should be some mechanic to make the round progressively more lethal as time goes on. I'm also a bit concerned that this round will likely result in a lot of ties from everyone dying simultaneously.
- 20 second timer means there isn't 75 seconds of waiting at the start of the round until people start dying, that becomes 50 seconds instead (still pretty long). It also means that once the killing slows down, it's not 45 seconds of waiting between each kill attempt. I understand that you'd be planting c4 more than once every 45 seconds, but 45 seconds between the arming and the detonation feels way too long for a fun round.
- There are some maps where this round will simply not be fun. A few that come to mind are crummy cradle and 67th way.
- Once enough people die, the killing is going to slow down dramatically. There needs to be some way to close out the round, or this is always going to end very slowly. Could be something simple like c4 radius gradually getting larger, or something more complicated like dead players coming back as zombies to hunt down survivors. That one isn't exactly thematic, but it would work.
I think this concept is pretty fun, but there are so many question marks in the execution that I don't really see it working out.