I personally don't see any problem with word "kafir". You can think others whatever you wish based on your personal beliefs or understanding of islam but someone is kafir don't mean you should go and kill them. Its not just about some sunni who consider shia as kafirs. I mean if you ask the opinions of shia about salafi /deobundi/wahabi then they would have the same opinions about them. The tension between sunni and shia only get worse when shia insult those companions of prophet muhammad(PBUH) whom Sunni consider greatest. The only sunni sect which shia consider good are brealvi.
secondly there is problem of definition of Muslim. Some scholar say you are muslim if you recite kalma which is there is no God except Allah and prophet Muhammad(PBUH) is the prophet of Allah. Now question arise if someone believe in this kalma and consider prophet muhmmad(PBUH) as prophet of Allah but don't consider him(PBUH) last prophet of Allah then will he be a Muslim? so then definition of Muslim will get change that you are Muslim if you believe in Allah and believe that Prophet Muhammad(PBUH) is the last prophet of Allah then qadyani get in equation who believe in this statement but consider mirza ghulam ahmad as second coming of jesus as well as imam mahdi whom others Muslims are still waiting? Are they muslim? According to pakistani constitution they are not muslim . Again what about those peopels who insult and declare some very close friends, relatives and wives of prophet Muhammad(PBUH) as naouzbillah munafiq? Should they consider muslim ? These are the questions which would have different answers from different peoples depend on their commitment with islam