like death and taxes

implacable inescapable merciless relentless unrelenting

adamant adamantine bound bound and determined compulsory dead set on dogged hard harsh hell bent on immobile immovable ineluctable inflexible ironclad locked-in mean business necessary no going back obdurate obstinate remorseless resolute rigid set in stone severe single-minded stubborn unappeasable unbending uncompromising unmovable unyielding

kind merciful nice sympathetic

flexible lenient remorseful yielding