It would probably require Trump to kill a kitten in a satanic ritual. Killing a person probably wouldn't be enough.
That's only half kidding, too. Impeachment is an inherently political act. For both parties to remove any president, it requires that sufficient numbers of both parties make the calculation that they are better off without that particular president. The Republicans have made the calculation that they are dead meat if they even criticize this president, so removing him is out of the question at this time. They are probably right in that. With the gerrymandering that has built most safe districts, Republicans are only really worried about being attacked from the right in the primary. Gerrymandering has happened for both sides, but Democrats are less easily attacked from the left, being a more amorphous position.
With Republicans, though, they've long had a fault line running through the center of their party, which makes them oddly susceptible to attacks from their right wing. The party is generally seen as the party of business...and social conservatives. These two positions don't overlap nearly as much as would be convenient. Some see the Republican party as being a party of fat-cats, and they certainly do have some, but the average Republican is not rich. In fact, I believe that they tend to be less well off than the average Democrat. I haven't seen any data on that, and it is pretty likely that the average independent (or non-affiliated) is financially better off than card-carrying members of either party. So, while the Republicans are seen as the party of business, they are not predominantly business. The business folks would prefer different policies from the social conservative folks. The two groups have different priorities.
Trump acts more like a classic Democrat when it comes to business and trade (protectionist, against free trade), but he stirs up the social conservatives like none other. It's a potent mix. Lots of Republicans in Congress probably loathe his positions (or else they've been lying for the last couple decades when it comes to their trade views), but if they oppose him, the social conservative branch, which outnumbers the business branch, will thrash them in the primary.
So, the Republicans won't oppose him unless he were to do something so appalling to the social conservative wing of their party that they stop supporting him. What he does in Ukraine...doesn't even register.

