S1. John [was] eaten by lions
First sentence,in my humble opinion, it means the event happened in past. The sentence 'John
has been eaten by lions' indicates that it happened in past and still remains as a fact until now.
S2. I [have] eaten my dinner
It means you mention that you have taken your meal at the point you spoke it out. Alternatively, 'I
had eaten my dinner when the bus arrived' emphasis that you have finished your dinner before the
arrival time of the bus.