If it is true that - during the lifetime of The Smiths - Joyce knew all along that he was only getting 10%, and then after The Smiths pled ignorance and went after Morrissey and Marr in the courts, then you can understand Morrissey and Marr's hatred of him. But it is an "if". Only the people involved know the truth, and they accuse each other of lying.
However.
The fact remains that the 40-40-10-10 split was not written down. It should have been, but it wasn't. In the absence of a written agreement, the default position (as far as John Weeks saw it) was that since there were four members of the band who recorded and performed together, it should follow that the recording and performance royalties should be split equally four ways.
Under this arrangement, Joyce and Rourke get an equal share of those particular royalties, but Morrissey and Marr continue to earn far more money from The Smiths overall due to them sharing the more lucrative writing royalties 50-50.
I do find it peculiar however that Joyce only happened to find out (if indeed he was oblivious before) about only getting 10% after The Smiths broke up. From 1982 onwards he had gone from being an unknown drummer to being a member of one of the country's most popular groups and earning good money into the bargain. How is it possible that neither he, nor anyone around him who was willing to tell him, knew what he was earning as a percentage of The Smiths' performance and recording royalties?
From memory, I believe Joyce claimed that it was only when he applied for a mortgage after The Smiths broke up that his financial status within the band became clear. That is when, apparently, he realised he was only on 10% and not 25% as he claimed to have thought before. But what documentation or information was he given at this point which specifically spelt out his 10% share that he was, somehow, not privy to between 1982-1987?
My take is that Morrissey and Marr f.ucked up by not getting Rourke and Joyce to verbally accept 10% and then sign a contract to that effect. I believe Joyce knew all along he was only getting 10%, but then - knowing nothing had ever been signed - saw an opportunity to plead ignorance and try to get more through the courts.
Morrissey and Marr were remiss in not getting a contract signed. But Joyce was morally wrong to seek money he knew he was not really entitled to.