A convicted felon who mugged and killed an 85-year-old woman at the Riverhead Public Library was sentenced to 25 years to life Wednesday.Last May, Garner Allen, 31, shoved Ruth Seybolt, of Flanders to the ground after snatching her purse at the library. Seybolt suffered brain injuries and died three months later. Prosecutors say a library surveillance video shows Allen following Seyblot for several minutes and leaving with her purse. The video also shows Allen coming back into the library to tell patrons that a woman fell outside and hit her head. A Suffolk County jury convicted Allen of murder last month. One of the jurors said it was the video evidence that clinched the conviction.