Of course I believe that God is "all good" as indicated throughout the Christian bible...
If you could jettison the OT you might stand a chance of making a plausible case for that. Unfortunately numerous passages in the OT portray God as a homicidal ultra violent monster, and ever since Marcion lost the argument Christendom has been stuck with that, unable to fully distance itself, even though it would prefer a wholesome loving God.
Marcion, though offering some interesting ideas, 'lost the argument' for very good reasons. His own version of the New Testament, though perhaps the first version we have of a NT 'canon', is also a very obvious early version of cherry-picking to suit what you want to believe about your God. In the first instance, it's all very well to decide which actual books should be represented in your collection, but when you start selecting only the 'nice' passages in the only gospel of which you really approve (in Marcion's case, Luke), then you've pretty well entered the area of making things up as you go along.
Marcion wasn't able to benefit from the methods of modern critical scholarship, which are able to provide reasonable guidelines for what Jesus may or may not have said, in a purely objective way. He was instead guided by a degree of gut-feeling and personal bias. Jesus sometimes appears as being a totally forgiving, loving chap, and suggests that we should turn the other cheek, and walk two miles with the bloke who asks us to walk just one mile with him etc. etc. There is of course another aspect to Jesus depicted in the NT, which suggest he thought God was in some ways as bad as he appears in the early part of the OT, and his own character in some places seems to reflect this violent, judgmental God. You can say "well, he can't have really said both things, because that's contradictory". And the evangelicals and the fundamentalists are keen to point out how these seeming contradictions can be reconciled. It is certainly difficult to reconcile them, and I see no reason to believe that all the sayings and deeds of Jesus in the NT are accurate reportage of the historical character.
However, Marcion's other great failing was his either/or attitude to the Old Testament. In much of the early part of the Pentateuch, God is portrayed as an absolute monster, however 'metaphorically' you read the text. But the OT, too is a huge compendium of differing ideas about God, and about what God supposedly requires of human beings. I refuse to accept that the God of Ecclesiastes is a 'monster' - in this practically 'Buddhist' text, God scarcely gets a look-in - he seems quite nebulous and conspicuous by his absence (In the book of Esther, of course, God is not mentioned at all). All three parts of Isaiah* depict God in different ways, but in general he seems much improved on the nasty war-monger who rampages through the Book of Joshua, or the vile homicidal sadist in Judges who still seems to be keen on human sacrifice.
By the time we get to the Book of Micah, God is almost altogether a reformed character, (apart from a few rumblings) and seems to me a rather superior to the God of Jesus, if you take all the gospels into consideration without 'cherry-picking'.
Marcion's work "The Antitheses" (of which we only know certain quoted passages) in which he outlines the differences between the two Testaments, seems to be a case of selective quotation of the most blatant and deceptive kind.
*Many scholars believe 'Isaiah' had a three-fold authorship.
