How do Christians holding some role of evolution defend against YEC that the many deaths required is adding blemish to God's character?

Augustine answers this question in The Literal Interpretation of Genesis book III chapter 25:

Someone is going to say: 'Then why do beasts injure one another, though they neither have any sins, so that this kind of thing could be called punishment, nor by such trials do they gain at all virtue?'

For the simple reason, of course, that some are the proper diet of others. Nor can we have any right to say, 'There shouldn't be some on which others feed.' All things, you see, as long as they continue to be, have their own proper measures, numbers, and destinies. So all things, properly considred, are worthy of acclaim; nor is it without some contribution in its own way to the temporal beauty of the world that they undergo change by passing from one thing into another. This may escape fools; those making progress have some glimmering of it; to the perfect it is as clear as daylight.

Note that he takes for granted the fact that there was death (in the animal world) before the Fall. He answers the question of the justice of animal death by pointing out how beautiful ecology is - how animals are designed to live together in a food chain in which some are the proper food of others, and through this whole process, the beauty of nature emerges. He did not know about evolution, but I think his words in these paragraphs can clearly also be applied to evolution - out of the individual chaos of animals living and striving and dying, a beautiful system and balance emerges, even a creative process.

He goes on to say that "all such goings-on in the animal world provide us human beings with plenty of salutary admonitions," i.e. that by us rational creatures observing the animal world, we can know that we ought to treasure our spirituality, otherwise we will simply live as animals do, which is not very desirable. If I may take a step beyond what Augustine says in this book, when God told Adam that "in the day you eat of it, you will surely die," it would have been clear to Adam the implication that eating the forbidden fruit would make him like an animal.

Continuing this chain of thought: If we suppose that animal death is bad on the basis of its similarity to human death, we actually demean our own status. We would thus put ourselves in the same category as the beasts. In reality, us being God's image-bearers makes us radically different, morally speaking, from animals. Death of animals is more akin to death of plants or even death of bacteria than it is to death of humans, because animals, plants, and bacteria are all non-image-bearing life forms. The superficial similarity of humans to beasts does not imply a spiritual similarity, so I really do not even buy the premise of the YEC argument on this point.

Speaking for myself, I think that there is a clear argument from design that animal death preceded the Fall: Go outside and look around. All the animals you see have features specifically designed either to kill other animals or to protect themselves from being killed. Many of the plants you see are filled with toxins to harm animals that try to eat them. The plants and fungi grow out of the corpses of dead animals. And, of course, it is difficult to imagine a stable world anything like ours where animals could reproduce but not die. Postulating that there was no death among animals before the Fall would require us to believe that God made huge alterations to world at that time, which is hard to reconcile with the fact that he finished creating on the 6th day.

If you haven't read The Literal Meaning of Genesis, I highly recommend taking a look. Augustine did not take the six days of Genesis 1 as six 24-hour periods of time. And yet Augustine was a staunch defender of a literal approach to Bible interpretation (see for example City of God book XV chapter 27) against allegorizers like Origen, and consistently insists that authorial intent must take precedence in hermeneutics. His reasons for not taking the six days literally are based on the text itself. He was obviously not motivated by modern science, nor was he motivated by the science of the day - he was perfectly willing to postulate, based on the Bible, things that were thought scientifically impossible by contemporary secular scholars. He also gives a shorter discussion of the topic of Creation in City of God book XI.