She knows why Wanda is doing this - why she wants this to happen. It's a side effects of sharing the same head space for however long.
And it makes her feel selfish, because she can feel how tired Wanda is. She loves Ian, but she's lived so many lives and this one is the most painful of them all and she just wants it all over, even if she uses the excuse that Melanie needs her own body back.
Melanie can't believe that Doc agreed to this though - this, right here, will be murderer, because this time the death is intentional.