1. Because two wrongs don't make a right.
2. Because, unless you happen to know Boris personally very well - which I strongly doubt - you have no way to tell whether your statement '"Boris" has little respect for anyone else' is true or or not. All you have to go on is lots of media reports. That you disagree with many of his policies doesn't mean he has "little respect for anyone else". That he has some apparent history of lying (and I think he probably has lied on some occasions) also does not necessarily mean he has "little respect of anyone else." (Although to be clear, I don't defend lying, and I tend to think worse of him because of it).
3. Because, even if, hypothetically, it turned out that Boris actually does have "little respect for anyone else", then by saying 'why should we have any respect for him', you're basically lowering yourself to the same standards.