Ok hear me out.
I don't think that TKs (technically RDMs) with C4 should incur a Karma Loss.
C4 does nothing for a considerable amount of time, during which time it is marked clearly on the HUD of all Ts. If you still can't help but get get killed by something that's clearly marked on your HUD than the problem is clearly with you.
Here's some recent examples to illustrate why i think it's a good Idea.
The other day i planted a C4 on a 2 minute timer. I then got killed 30 seconds later. The C4 went off 90 seconds later and killed a T who was standing next to the C4.
Last night, the T side had done incredibly well, there were 7 other Ts alive and 1 innocent. I'd planted a C4 at the beginning of the round in the tester room. I watched in amazement as 6 out of the 7 other Ts walked right up to the tester room with only 10 seconds left on the c4 clock (and warning them that there was a C4 there) and promptly got blown up, causing me a huge amount of karma loss.