In the Season 2 episode "Go with Me", Marceline says that she likes Finn, but she isn't in love with him. But if she didn't love him, then why did she scare Finn first and then told him that when he wasn't scared off by her. Could she have told Finn that first? This shows that Marceline might like Finn more than she want's to admit, but since he and Jake are mortals she is afraid of loosing them (like in "Heat Signature") even though she could turn Finn into a Vampire.