I feel Bill Bailey said it best
'Half empty or half full? I don't know - depends what's in the cup. Could be beetroot, some gravel or small stones, perhaps even blood. Is it being used in a sacrificial context?'
This is a very narrow look on a much broader question, I feel we need more information to make a balanced judgement, are there any extenuating circumstances involving the glass we should know about. What is in it and in what context is the liquid being used. Is there a malicious plot involving the liquid if so it is half full and a half too much imo. However if it is a glass of blood on the way to save someones life (unlikely as that is please run with it I having a crazy rant it's allowed) then it's half empty and we need to fill it up.
I feel unable to answer this at the present time due to lack of history of the aforementioned glass and liquid and feel that it must be brought to light the true nature of the glass before I can make a informed decision.