The key word to get the meaning of this is “assez” which translates to “enough”.

So this sentence is literally:

“We cannot be absolutely sure that X has killed Y, but we can be sure enough about it so that it’s sure.”

Strange sentence, indeed.

There is also a double meaning behind “sûr” as it can apply to my degree of knowledge (“J’en suis sûr” -> I’m sure of it) or to the probability of something I’m talking about (“C’est sûr” -> It’s certain). The use of the verb “soit” creates a link between the two aspect of the word: our internal knowledge and the intrinsic property of the object.

The meaning of the sentence is obviously ambiguous so it can be understood as:

1) We can be sure enough that X killed Y to affirm it with a low probability to be actually wrong.

2) Their is a degree of knowledge concerning the fact that X killed Y above which X has literally been killed by Y!

A strange way to deal with the presumption of innocence indeed…