Welch voluntarily left the band in '73/'74. After a successful solo career, he shot himself in 2012 after back surgery and being told by his doctor he was likely going to be an invalid. There is nothing to suggest that his suicide had anything to do with how he was treated by Fleetwood Mac.
Now firing Lindsay Buckingham was inexcusable. To think that Fleetwood Mac can exist in his absence is laughable. Stevie Nicks was certainly a hottie back in the day, but she is a world-class bitch.