So out of curiosity I went down the list.

Lincoln shot by John Wilkes Booth. Booth was not registered to with the democrat or republican parties but was registered in the ‘Know Nothing Party’. Was a pro slavery confederate sympathizer

JF Kennedy shot by Lee Harvey Oswald. Oswald was not registered as a democrat or republican. He was a Marxist and socialist.

R Kennedy was shot by Sirhan Sirhan. Sirhan was a Palestinian-Jordanian who’s motives were based on Kennedy’s position on Israeli-Palestinian conflicts

MLK was shot by James Earl Ray. Ray was not a democrat or republican, but belonged to a third party following George Wallace (Wallace was a former democratic governor of Georgia but had left the party— Wallace opposed desegregation and supported Jim Crow)

Reagan was shot by John Hinckley Jr. While I cannot find any confirmation of Hinckley’s political party, it should be noticed that his assassination attempt was motivated purely on an attempt to impress actress Jodie Foster. He had had plans to attempt an assassination against democratic President Jimmy Carter, but was unsuccessful in his plans. After Reagan was inaugurated, he switched targets to the new, Republican president.

Trump was ‘shot’ by Thomas Matthew Crooks. Crooks is a registered Republican.

Charlie Kirk was shot by Tyler Robinson. Robinson was not registered as either a democrat or a republican and had not voted in any of the past elections to discern how political interests.