I expect it was something more like manslaughter than an intentional act of murder (i.e tried to drug her but she was already high as a kite on something that reacted badly to whatever they used), or even her drowning on her own vomit while (semi-)voluntarily in their company, and then the little weasels dug the hole deeper by disposing of the body and trying to cover it up instead of walking away and playing dumb afterward, which would probably have worked out better for them.