The Death Eaters can't all be pure-blood, there aren't...

The Death Eaters can't all be pure-blood, there aren't enough pure-blood wizards left," said Hermione stubbornly. "I expect most of them are half-bloods pretending to be pure. It's only Muggle-borns they hate, they'd be quite happy to let you and Ron join up" "There is no way they'd let me be a Death Eater!" said Ron indignantly...."My whole family are blood traitors! That's as bad as Muggle-borns to Death Eaters!" "And they'd love to have me," said Harry sarcastically. "We'd be best pals if they didn't keep trying to do me in.