Was the Williams team legally found guilty for Senna's death?

Yes. But nobody paid in the end.

This is very little known but yes, the Williams team was finally found guilty for Senna's death. 
"The Williams team was entagled for many years in a court case with the Italian prosecutors over manslaughter charges, ending in a guilty verdict for Patrick Head. The Italian Court of Appeal, on April 13, 2007, stated the following in the verdict numbered 15050: "It has been determined that the accident was caused by a steering column failure. This failure was caused by badly designed and badly executed modifications. The responsibility of this falls on Patrick Head, culpable of omitted control". Even being found responsible for Senna's accident, Patrick Head wasn't arrested: in Italy the statute of limitation for manslaughter is 7 years and 6 months, and the final verdict was pronounced 13 years after the accident."

Ayrton Senna Memorial at Imola, San Marino, Italy

Ayrton Senna Memorial at the circuit of F1 in San Marino, Imola, Italy

 "I’m not a big fan of May 1. I know what to do each year on my wife’s birthday, our wedding anniversary, on Christmas, and all of the other major dates circled on the calendar, but the annual reminder of Ayrton Senna’s death on May 1, 1994 tends to leave me a bit confused and emotionally conflicted. It's been 19 years since his crash and subsequent death at the San Marino Grand Prix—more than enough time to have gone from mourning his loss to celebrating his life—but there are still tinges of sorrow that surface each May.

As much as I’m thankful for all he accomplished and left behind, I’m also somewhat selfish in wanting to have seen more. I feel the same way about the late Greg Moore and Dan Wheldon. Having witnessed their brilliance for only so briefly, living without it has left an immeasurable void that, if I’m honest, will probably remain unfilled. What I’m also left to consider each year on May 1—and I imagine I’m not alone—is how big of a role Ayrton Senna played in my life. Like millions of other Formula 1 fans, I dove headfirst into the sport in the 1980s. Despite my appreciation for many of the World Champions in the field, I was immediately drawn to Senna, a driver whose otherworldly talents were so perfectly balanced by his many flaws. He was painfully human —the most relatable driver on the grid, I reckoned. I digested every bit of news and information on Senna that was available from magazines like Autosport, On Track, and MotorSport (during those pre-Internet days) and videotaped nearly all of his F1 races (once I was old enough to get a job and pay for cable, that is).

I spent countless hours arguing with co-workers, bosses, and drivers about his superiority to Prost, Piquet, Mansell, and the rest. He was my hero, and my worship of him continued into the Nineties, adding Schumacher and Hill to the losing column in my superiority debate. By the way I miss those heated roundtables but remain convinced that I was always right... And then it came crashing down for me on live television in the early hours of May 1 at a hotel near Laguna Seca.

The outcome of his crash was obvious at the time, and although I was in shock, the announcement of his death that came over the public address system in the Monterey paddock wasn’t unexpected. Following Senna was incredibly personal for me and made his death hard to reconcile. Again, I can’t say if my attachment to him—one forged from half a world away—was unique, but every year on May 1, I’m taken back by the flood a memories—of who I was at the time, of what he meant to me, and of how my appreciation for him has matured."

By Marshall Pruett