Probably, why did Derek Chauvin not want testify ?  was that a mistake or not? why?

I don't think it will make any difference.

damned if you do, damned if you dont.  he would have been tripped up had he testified, but by not testifying, it gives off a strong impression of guilt. 

I think the defendant rarely takes the stand. Could be wrong about that.

Derek Chauvin actually pleaded the 5th Amendment, refusing to testify on the grounds that it might incriminate him.  That's why.  In other words, if Chauvin answered questions truthfully, he was quite concerned that it might well prove his guilt.  And if he lied, he would be guilty of perjury.

He has an air of confidence and superiority even now despite the circumstances.  Testifying would be helping the prosecutors.

No, the prosecution would have dragged up everything in his entire life he ever did that might seems dodgy. The defense always suggests you not testify because it shifts the burden of proof and makes the jury think about whether or not they "like" you as a person. What they should concentrate on is the evidence only.

Because when you get a criminal on the stand he is wide open to be questioned about his motives, his past and his reasoning.

Chauvin knew Floyd before this arrest.

That would come out, too.

He only hangs himself if he gets on the stand.

The issue isn't him testifying, but him being on the stand also means that the prosecution could grill him on cross, which is what he wanted to avoid.

No mistake, that was the smartest thing he probably did in his entire life. However, that still might not save him.