Why should G.P not take responsibility for not mistakes

I am so angry . My son in law age 31 with 2 children went to G.P IN June complaining of migraine never goes to doc never off work told eye strain . Was at G.P twice a week for four weeks . Putting his head of buckets of cold water to try to ease pain .My daughter was begging doc to send him to hospital no it was only eye strain went to A E to be told I know a migraine when I see one . After 6 weeks which that lad went through so much pain got hospital appointment. Brain cancer half his skull taken away and has spread rest of body .We know he only has weeks .If any one else made that mistake they would be out of jobs.And if takes my last breath I will fight this tooth and nail