i suggest you read the novel if u want understand their characters more. Adolph' love was geniue and heavy, that what make it even more sad, because he failed him badly and broke everything he promises. Lewis describe Adolph was like "child on adult body", Adolph was childish and selfish, his only exception was Lewis.
honestly the start of tragedy was probaby when Lewis quite his job because Adolph get jelous when he talking with other. and since then like a kid that lost his parent surveillance, Adolph make "mistake", he cheated when drunk. when Lewis find out he having affair, he just keep silent. otherwhise if only he get angry and confront Adolph, he probably never cheat anymore for real. there more to it but it been while i read the novel so i forgot the specific. basically although Adolph at fault, Lewis actually have the chance to fix their relationship before it get worse, but he never took it.
how much Adolph love Lewis? in fact he love Lewis more than he love himself. when he found out Lewis sickness, he was so regretfull he hope he was the one who sick. for Adolph, Lewis was like air and water, the existance you take for granted but when it dissapear it kill you. Adolph actually have very strong dependency on Lewis, that why he so broken down when Lewis left him and so in denial when he died. he never notice Lewis bad condition because he take his existance for granted and the posibility of Lewis ever leave him was never crosing on his mind.
also the in the novel actually there 2nd part of letters. on the end of his letters he wrote that: he, just like Lewis will wait for 4 years, and if Lewis still not back he will be the one come to Lewis instead. so yeah, its sugest that Adolph commit suicide after waiting 4 years.