First, if the ATCO in question was having such a 'bad day', perhaps he/she was unfit for duty. Clearly whatever may have been causing his/her 'bad day' was hazardous to a safe operation.
Second, the RTF is NO place for the alleged behaviour. That in itself is another recipe for distraction and possibly a further incident.
Third, ATCOs and pilots should always remember that the person at the other end of the radio may be inexperienced; having technical difficulties with the radio; have just been distracted [rightly or wrongly] etc.
Fourth, 'Say again slowly' is part of the standard RTF phraseology....not an indicator that there is a dullard on the other end.
Fifth, the holes in the cheese can line up so easily, they don't need any help!
Finally, it is 'safety first',......even when the person at the other end of the radio is an annoyance!!