Well arn't Reapers also synthetic life forms? Technically someone must have created and programed the Reapers to continue the cycle. So really you need to blame whoever created the reapers. But the reapers probably killed that race off already!
And even though the Geth and Quarians can be at peace (Remeber one option has the Geth killing all the Quarians), war can still break out again. Also nothing prevents the creation of new synthetic life forms