The seminal event in parshat Ki Tisa is, of course, the egel. It seems quite astounding that so soon after matan Torah at Har Sinai, Bnei Yisroel violated one of the fundamental prohibitions in the Torah - indeed, the second commandment - not to worship a graven image.
To be sure, the consensus of the commentators (Ramban, Ibn Ezra) is that Bnei Yisroel did not view the egel as a deity. Rather, concerned that Moshe had died, they sought an alternative intermediary to worship Hashem. Yet, Moshe repeatedly refers to the cheit ha'egel as "chata'ah gedola" - a grave sin (Shemos 32:21, 32:30, 32:31).
In other contexts, the Torah makes clear that the worship of graven images is a more severe sin than any other. In the sefer Parperaot LaTorah, Menachem Baker makes an interesting observation about the language in Ki Savo 27:15: "Arur ha'ish asher ya'aseh pesel u'masecha - Cursed is the man who will make an idol." He questions why the pasuk is phrased in the future tense (asher ya'aseh - who will make), instead of the present tense - "arur oseh pesel u'masecha - cursed is the man who makes an idol." The use of the future tense makes the pasuk inconsistent with the rest of the curses, which are all phrased in the present tense - e.g., "arur makleh aviv v'imo - cursed is one who strikes his father and mother," or "arur masig gevul ray'ayhu - cursed is one who invades the boundaries of his friend." Why the different tense for idol worship?
The answer is that with all other transgressions, Hashem punishes the actual sinful act, but not thoughts to commit the sinful act. As per Kiddushin 40a: "machashava ra'ah ein hakadosh baruch hu metzarfa la'ma'aseh - Hashem does not deem the sinful thought an action."
However, when it comes to avoda zara, the mere thought to commit such a sin is deemed equivalent to action, and warrants punishment. Hence the language in the pasuk - "asher ya'aseh" in the future tense, and not "asah" in present tense - i.e., once the thought to commit avoda zara is formed, it's equated with action and culpability attaches.
So to frame the question: why does the Torah consider avoda zara such a severe sin to the extent that sinful thought is equated with sinful deed? Answering this inquiry will, in turn, help us understand why the creation of the egel - a tangible intermediary to Hashem - was such a dangerous development.