The most important point about Guede - by far - in this regard is his hand print found, made by Kercher's blood, on the pillowcase underneath Kercher's hips, coupled with his epithelial DNA found inside her genital area.
Those two pieces of evidence are extremely important in the case against Guede. The DNA proves that he had some level of sexual contact with Kercher's genitals, almost certainly close to the time of her death. The hand print is even more damning, in that it definitively places Guede at the murder scene within a defined time frame - the time between the victim being stabbed and the following morning.
Guede and his lawyers knew very well indeed that they couldn't hope to argue that Guede wasn't there, nor that he'd had sexual contact with Kercher. The evidence to the contrary was strong and reliable. That's precisely why they had to cook up a story where Guede could a) claim prior consensual sexual contact with Kercher, and b) place himself in Kercher's room shortly after the stabbing, all while claiming that he himself had nothing to do with the actual attack and murder.
The DNA on the handbag (purse) and on Kercher's Adidas tracksuit top are really neither here nor there in terms of proving Guede's presence and level of sexual contact with Kercher. As I've argued many times before, Guede could (and would) have gone down for murder based on nothing more than his palm print, his DNA in the genitals, and his post-murder behaviour (the trip out dancing later that same night, plus the flight to Germany).
