You do have to have a dim outlook on your prospects of winning to agree to pay out 10 billion.
For executions there are additional things as well beyond the regular criminal trial.
Which isn't to say that a civil trial verdict isn't accurate to some extent - it just isn't absolute proof. There's some evidence in scientific literature about roundup's carcinogenicity at high doses (like what a farmer or landscaper would commonly encounter) and other various health issues at high dose. The big question is what happens at low doses and if it's impactful. The science is still out on that (and the alternative of lower agricultural output per unit of land area isn't really palatable from both a starvation risk perspective and a climate perspective).