India is as solid as it is because its former dozen warring states (remember, it was not always a single country) got head-banged by the Brits during the Imperial era -- if that job had been finished, rather than abandoned as the British Empire fell apart, we might not have today's conflicts, or at least they'd be on a smaller scale
That's an interesting example, but I don't think colonialism alone can build nations.
In India's case, the British didn't leave India with Indians marching into the sunrise in patriotic unison. Oh sure, the post-Independence honeymoon lasted a year or two, but secession and splintering back into a bunch of states was never really off the table for the first quarter-century after Independence, and it was especially touchy because of the partition that accompanied Independence.
Hell, there are sporadic movements *now* but they're not considered serious threats any more, largely because India's political system has evolved to the point that even very small sections of society can get a voice in the political process.
So while Britain did do a lot to create a modern nation-state, I think post-Independence Indian society deserves a lot of credit for seeing that, to misquote Ben Franklin, it's better to hang together than be hanged separately.
The bigger point is the British did all of this in many places, including Africa. The problem is, as soon as they left, the societies fell back to their old patterns of conflict. I'm not sure more Imperialism would have fixed that.
Imperialism may be "evil" if your small state is the loser, but in the long view it appears to stop more trouble than it causes.
Imperialism is evil because imperial powers don't come into colonized countries even wearing the fig leaf of spreading democracy or development, they come to use the resources of the colonized country the best they can. In India's case, the British exploited India's farmers with exploitative taxes (even during famine years) and took as much of India's natural resources (mined metal, timber, etc) as they could given a 19th century supply chain. Whatever problems arise *after* an imperial power leaves, at least they are problems brought upon by the people themselves, not a foreign power.