Which part of his behaviour in this email is unacceptable in particular? Let’s look at it:
> Mauro, SHUT THE FUCK UP!
Okay, so he tells Mauro to stop talking because he apparently thinks that what Mauro was saying is wrong. To make very sure that what he wrote is read, he puts in fancy caps and some light swearing, which, however, is not directed at anyone in particular. A more polite version of this would have been ‘No.’, but I don’t see any bullying/abuse.
> It's a bug alright - in the kernel. How long have you been a maintainer? And you still haven't learnt the first rule of kernel maintenance?
It appears that this guy violated some rather important rule of kernel development which he should have known given his experience. Again, no name-calling or directed swearing whatsoever, however. Again, a more polite version could be ‘We don’t break userspace, and you know that.’, which doesn’t look particularly different to me.
> If a change results in user programs breaking, it's a bug in the kernel. We never EVER blame the user programs. How hard can this be to understand?
I would wager the guess that similar problems pop up from time to time and Linus is getting tired of seeing them all the time, hence his question. Again, no name-calling at all, also no abuse, but a (justified, though maybe slightly aggressive) question.
> [technical details]
> Shut up, Mauro. And I don't _ever_ want to hear that kind of obvious garbage and idiocy from a kernel maintainer again. Seriously.
Again, no name-calling of the maintainer, but only an assessment of what he wrote, which, to Linus, appears very wrong. Again, more politely, this would have been ‘No.’
> […] And you've shown yourself to not be competent in this issue
This is the first personal ‘attack’, however, it is still only aimed at the competence rather than person, and if the above holds true, is yet again entirely justified. Is this the line with which you had a problem?
Linus then goes on to explain again why this particular idea was ‘TOTAL CRAP’, with some more colourful adjectives regarding the patch. Again, no abuse.
> The fact that you then try to make excuses for breaking user space, and blaming some external program that used to work, is just shameful. It's not how we work.
This could be interpreted as somewhat personal, though it likely only means ‘I expected better and am disappointed in you’.
> Fix your f*cking "compliance tool", because it is obviously broken. And fix your approach to kernel programming.
Note that the ‘fucking’ is for the ‘compliance tool’, which apparently is broken; the comment regarding kernel programming is again directed at their competence rather than person, so I don’t see any particular abuse there, either.
The remaining question is then, why didn’t Linus just write something along the lines of ‘No. / Don’t break userspace. / No. / No.’? Likely because then someone would have called him out on not going into details or because they got the same email regarding three small mistakes and a question and now don’t know whether this issue is a similar ‘small mistake’ or something more problematic.