This entire story is being concocted by a ***proven*** liar who is trying to save his own ass...

Very likely.
The jury will be aware that the guilty always lie at their trial anyway, and will put little or no weight on his assertions.

It is not at all unusual for a killer to plea self defence.
Guilty or not, the jury must acquit unless convinced beyond reasonable doubt that the deceased did not attack the accused.

Apart from the conflicting evidence, they will also need to assess the position of the deceased on a scale from violent thug to gentle saint.
They will need evidence of his past to do that.