First principles of supervisory management: "Focus on the issue not the person"
One of the first principles of criticism in supervisory management is to focus on the issue not the person.
'You did this wrong, let Bob show you how to do it right' instead of the childish 'You're an idiot.'
<< There is no nice way to tell someone they suck. >>
That is the crux of the matter -- the incorrect concept of what a human being is and what it would take for a human being to actually suck.
You can be a terrible programmer and/or a terrible tester -- those things just mean you are a terrible programmer and/or a terrible tester.
Programming and testing are just things people do -- not what they are.
A person is a lot more than just what they do at work or for a hobby.
So there is NEVER EVER A LEGITIMATE REASON TO TELL SOMEONE THEY SUCK.
Worst case, if you've tried coaching them, if they still don't get it, you can tell them, "You just are not working out at this job. But I'm sure you will find success in some other career.
Linus Torvalds does not suck big time. But he is simply not working out as a project leader and "front man" for Linux.
Look at Windows -- thousands of people have been lead by Linus Torvalds to create an OS to replace the pathetic product Windows -- and he can't take market share from Windows. That is a hallmark of marketing and front-man FAILURE (true, one shared by Apple). Linus is doing a really piss-poor shitty job at his own job.
But that does not make Linus a failure, a looser or an idiot.
It does not mean Linus Torvalds sucks big time.
I'm sure Linus would have great success in some other career, perhaps coding or backroom QC guy.